In order to do a lot of tactical exercises the Chess Tactics Server will be a very interesting website. At the moment about 24,000 tactical chess problems from trivial to ambitious are offered at this site and have to be solved against the clock.
The site also uses a kind of rating system to enable you to measure your improvement. You can sign in completely anonymous as guest to give it a try, get familiar with the chess board interface or browse through some problems.
After registration you are able to measure your improvement by means of the mentioned rating system.