Currently the Chess Tactics Server has 23,803 tactical chess problems with a degree of difficulty ranging from trivial to ambitious. There is a degree of challenge with time control as well as the tactical puzzles are timed. Your tactical performance will be measured by the Glicko rating system. The problems are rated too.
You can sign in completely anonymous as guest to get familiar with the chess board interface, browse some problems or just look around.
If interested, you'll just need a valid email address to register and join the Chess Tactics Server community. If you know of any other solid online tactics servers, feel free to share them in the comments.
Link - http://chess.emrald.net/
Other Tactics Server as per Comments: http://www.chesstempo.com
jrobichess makes chess videos and has a chess blog along with a personal chess site at http://www.jrobichess.com.
