What is there to say? Tricks Tutorials is the classic. Great and funny tutorials for lots of basic tricks. There are also very smart articles to help people with their tricking, nutrition, and flexibility. There's also some off-topic content that makes the site all the more fun. Tricks Tutorials for the win!
The biggest Break Dancing community on the net. Well, could there be bigger ones? Who cares! This site is plain, simple, pretty, and has what you need easy to find without all the stupidity. Bboy.org is cool, so go check the site out.
Beast Skills is a site based mostly around gymnastics-type strength/skill development. Hence the name, BEAST skills! The strength poses and tutorials for various things will probably come in handy for the aspiring athlete.
This is a site which has lots of drills for lots of gymnastics skills, as you can probably tell by the name. People learn many ways, and this site has many different drills that one can perform that will help them to be able to land something. I personally love simple drills which help build ones awareness, so I can say you may find this site very useful indeed!
Exrx's muscle directory rocks! It has information on all the major muscles in the body, as well as workouts and stretches for each of them. This is a great resources for people who want to find useful ways to help condition their body. More than that, the website is a stupendous resource for all kinds of exercise and diet information. It is truly one of the best references on the web.
The home of Team Loopkicks and the Loopkicks Camp! These guys are awesome. Plus, the hold annual camps for many trickers to attend and have fun. Plus, the individuals trickers are just awesome! Their website has plenty of cool stuff, check it out.