Developing for Linux can be a lonely life. If you have the need to feel connected to a larger commuity of developers, or you just need help with a tricky part, then these sites can help you out. After all, no matter how good a developer is, he cannot be an island unto himself. Of course, if you are that good, you can still enjoy the company of your peers. You could also spend quality time impressing others with how prolific your skills are. Maybe, you will get a chance to do both. Enjoy the listing.
Kernel
Granted, you can't get into this site unless you are coding for the Kernel, but if you get in you will have a lot of fun. Just remember that it never hurts to ask members to recommend you, if you want to get in.
Sorce Forge
This site is your go to for all things Linux code. You will be able to find just about anything that you want (or need) on this site. It has something for every coder and every language under the Linux sun.
The Linux Foundation
This site is more about companionship and collaboration, than it is about specifics of code. Any developer who wants to network, or who has a family full of glassy-eyed zombies from listening to you talk about code, will want to check this one out.
Mozilla
Sure, they spend a lot of time talking about propriatry projects, but they are a lot of fun anyway.
Linux Game Development Center
Come on, you know that you want to have fun. Go ahead, and make a game.
IBM Developer Works: Linux
If you are in search of a more structured and professional enviroment, then Developer Works may be the way to go.
Linux Dev Center
This site has a wide range of articles written by experts that can help developers of all levels.
The LInux Magazine Site
Sure, you could buy the magazine, but you can get a lot of the same information for free on the site.
Fresh Meat
Its a good site for the newbie to intermediate developer. It features both articles and a blog with fresh content every week.
GNU Coding Standards
If you need the very basic information, or just a refresher of the standards, then this site can help you out.
Enjoy the site, and have a great time. Just remember the proper forum ettiquette.