See a more updated version of this answer on my blog
Here's my complete analysis of other programming websites:
The thing that must be mentioned first is IMHO, Stack Overflow is the best Q and A programming site. Tons of other sites offer similar, but not as good, Q and A experiences. The thing Stack Overflow lacks is a tutorial submission feature. A great site for this is The Code Project. Between The Code Project and Stack Overflow, there isn't anything else you need. Stack Overflow crushes every other site in posts per day and visits per day according to alexa.com.
First: what sets Stack Overflow apart. Stack Overflow is not forum based, where you ask your question in a particular forum, like the C# forum. Stack Overflow is tag based, which I think makes it much easier to look at, find, and search through questions. Stack Overflow is the only website I've found like this.
Second of all, most other programming websites I've found are more, "social", than Stack Overflow. They have personal messaging where users can message each on the site, and there's a lot more emphasis on user profiles.
Third, Stack Overflow is run by the community. Most other websites are run by administrator approved moderators, which have to each by appointed specially. Stack Overflow has at least 175 users with 10000+ rep, who all help moderate and run the site. other sites have maybe 10 appointed moderators and administrators, and the site is run by them. Being run by the community gives users a chance to really make a difference in Stack Overflow more than they can on any other site.
My unofficial tests have shown that Stack Overflow has the most questions and posts per day than any other site, and also the fastest responses. Don't worry about how many users a site has, what really matters is how much activity in questions and answers is on the site. I don't mean to squash smaller sites, but it's a lot nicer when your questions get answered faster and you have more questions to answer. alexa.com, a great site for comparing traffic across different sites, shows that Stack Overflow clobbers all of the others.
Stack Overflow is a relatively new programming site (less than a year old), but it has exploded in popularity. Most of these other websites are several years old, and Stack Overflow sort of has a lot of "cutting edge" technology and ideas driving. Well, at least newer ideas. ;D This is one of the reasons it's so different.
Most programming sites have some system of "reputation". I like Stack Overflow's the best, it works better than the others. All of the websites in the "worth checking out" section have some type of reputation system (although, no matter how much "reputation" you have, it doesn't actually let you do anything, it's just bragging rights. Long live Stack Overflow's community based operation :D). Many of the others don't, only counting your posts. I prefer at least having some record of the people I've helped (so that I can show off). But Stack Overflow lets me actually do stuff with my reputation.
Good:
daniweb.com: broad topics, not just programming and web development, but many other computer issues. It is worth checking out. Daniweb and dev shed are both very broad computer sites, taking questions on software design, hardware, web development, etc. No tutorial submissions.
codeproject.com: IMHO their forums are a little messed up, but The Code Project's main "point" is posting articles that you write about coding. It's definitely the most popular site for this, and you aren't limited to just tutorials, "articles" encompass a broader range. I recommend using Stack Overflow for questions and The Code Project for tutorials; you won't need to go anywhere else.
dreamincode.net: programming and web development help. It is a large site. They have a nice tutorial submission mechanism, but The Code Project's is much bigger. You can't edit your posts after 2 days, you're only hope to moderate the site is to be promoted by the owner, which barely ever happens, no up or down votes, not a very good rep system.
devshed.com little smaller than the others. It has been around for awhile.
forosdelweb.com hands down best programming site in Spanish.
Nothing special:
There are some websites that are web development only (just server side and client side technologies, no software development) These have a narrower view and tend to have more focus on web development and more posts about it.
Web development sites:
There are many sites that are specifically directed at one language or technology. Tons of these sites exist, though they are usually smaller but have more focused scope. I will only list a few here as they are extremely easy to find through search engines.
Java: Sun Forums (now Oracle Forums)
C#: csharpfriends
PHP: PHP Freaks Forums