Craigslist is great for finding so many things locally, such as rental listings and roommates, used goods and cars, job listings, and even personals ads and dating. I'm a huge fan of the site, and over the years I've used it to find roommates, sell cars, and find lots of ridiculous deals. I've also built up a list of other sites which do certain specific things better than Craigslist, and I've compiled them all here:
Buying and selling used stuff
Backpage: Backpage is not very well known, but its huge - and its probably the most similar site to Craigslist - it has the same non-nonsense, spartan look and feel. In most cities Backpage has a huge number of listings in every category from jobs to used goods to cars to rentals to personals. Backpage is best known for car listings - they usually have far more auto listings than Craigslist. The best thing about Backpage is that its completely free - even for job postings (which Craigslist charges for).
eBay Classifieds (formerly known as Kijiji): Did you even know that eBay has a huge classifieds section, separate from its auction service? Its true, and its a great alternative to Craigslist for any type of used products. What I like about it is the big pictures. Every item has a large image so you can easily browse through nice big pictures rather than clicking through tiny text links.
Oodle: Oodle is also very big and has thousands of listings in most categories. Oodle is best for real estate, they typically have far more real estate listings than Craigslist or Backpage. Plus you can see photos of each property.
Recycler: Recycler began its life as an auto trading site, but has expanded to host listings for pets, real estate, and used goods. Its smaller than Craigslist, Backpage and Oodle, but it is growing quickly. Posting on Recycler effectively posts on a bunch of other sites and print publications including AOL, Wal-Mart and Penny Saver so its a good place to post an ad.
Finding rentals and roommates
ApartmentRatings: This site is good for finding reviews on apartment buildings. The coverage is a little thin in some cities, but its a good place to check for top-rated apartments.
Yelp: Surprisingly, Yelp (which is better known for its restaurant reviews) is also a great place to get the scoop on apartment buildings. You can typically find at least a couple comments that previous renters have left about an apartment complex. Again, its a good place to start to do some research.
Apartments.com: If you know exactly what you're looking for, like number of bedrooms, pets allowed, type of carpeting - Apartments.com is a good search tool. It has a huge number of apartment listings across the nation, and it has a nice tool which asks you what you're looking for and gives you a filtered list of results.
Buying and selling cars
AutoTrader: One of the biggest sites online for buying and selling used and new cars. Lots of listings, with nice search tools that let you filter down to what you want. Probably the better choice if you're looking for a new car. Selling a car on AutoTrader can be be faster, with a serious and prequalified audience of buyers. AutoTrader also has some nice pricing tools to help you price your used car for sale.
Yahoo! Car Finder : Yahoo! has a large listing of cars and partners with many other providers to give you a huge selection of car listings in almost every city in the US. Also better for new car buyers, and it has a great search and filtering tool to let you select the exact options you want to narrow down your choices.
Lease trading sites: One interesting concept I came across reading this WSJ article was the high number of people who are trying to get out of their auto leases early. Some sites have cropped up which connect people wanting to dump their leases with people who are willing to take over the payments, usually with the "seller" providing a "buyer" with a nice cash incentive for taking it over. This can be a great way to get a good deal on a slightly used car. Two sites which provide this service are Swapalease and LeaseTrader.
Other classifieds sites: Backpage has greater coverage in auto listings than most other classifieds sites, including Craigslist, so this is a good place to start. Classifieds.com also has good coverage for auto listings, as does Recycler.
Personals and dating
OKCupid: OKCupid is free and feels more like a social network than a dating site. You sign up and create a free profile and they provide plenty of ways to flirt, wink, and play games with potential dates.
PlentyofFish: Similar to OKCupid, another free dating site.
Zoosk: Zoosk is not completely free, but it allows free members to search and browse profiles, and it has a virtual currency system that lets free members pay for some premium services. Zoosk is huge and you'll find lots of members even in smaller US cities - Zoosk also has large memberships in dozens of countries across the Americas, Europe, and Africa - so its a good option for people outside of the US.
Mingles: Free dating site similar to OKCupid. Also has a lot of members, and is a good alternative to Craigslist in many areas.
Finding skilled service providers (programmers, writers, designers)
eLance: If you're looking to hire a writer, designer, or programmer, you'll have a better chance of finding a qualified professional via eLance. eLance lists service providers from all over the world, and is supported by a comprehensive client review system so you can choose people with good reputations.
Odesk: Odesk similar to eLance and is also good for finding programmers, writers, and designers. Odesk is particularly well known for being a good site for hiring graphic designers and administrative support people, including virtual personal assistants.
Hiring home contractors
Estimate Magic: Great site which gives you free estimates from multiple contractors for your job. You simply provide some details on the job you need done, and you will receive 3 or 4 free estimates from available contractors. Estimate Magic also provides certifications that their providers are insured and licensed. Much easier than hunting and calling people on Craigslist, many of whom are unlicensed.
Calfinder: I used Calfinder to find a window repair contractor and really liked the service. Calfinder's site is nice because it helps you through the remodel and construction planning process - it helps you understand the steps involved, what you need to think about and plan for ahead of hiring your contractor. It even has a lot of great ideas for interior design and remodeling. On top of that it makes it easy to enter in the job you need done and you'll get several free estimates from providers in your area.
Service Magic: Site focused solely on connecting you with the right home contractor, including electricians, carpenters, window repair pros, handymen, and more. Service Magic is easier than Craigslist since you just have to put in your job description (what you need done) and the site will do the rest and connect you with contractors who are available in your area.
Angie's List: Angie's List is not free (you need to pay to view their reviews) but I'm including it because its such a popular option for people hiring home contractors. The site provides in-depth reviews from prior customers on all of their providers, so you can search for who you need then do a background check on them before hiring them. The reviews are detailed and honest, and for me I found it indispensable for hiring a home contractor for a major job.
Finding ridiculous deals
One aspect of Craigslist that I love is that sometimes you can find ridiculously good deals on items there. Its part of the fun of hunting around the site for stuff. Here are a couple other sites I've come across recently which offer new and fun ways to find ridiculous deals.
Swoopo: This interesting site is based in Germany and has an innovative auction bidding model for items like electronics, laptops, TVs and more. Its not too uncommon to be able to buy a high value item like an LCD TV for $100 - $200. Swoopo is part auction, part game - you buy "bids" for US$0.60 each and use those bids against other users and a timer clock to buy products. Check out the site for more details on how it works.
Oohilove: Similar to Swoopo, but for luxury fashion items such as handbags and shoes. You can get top fashion names for ridiculously cheap prices here.