Most everybody probably knows about this site, but just in case you don't, it's an excellent site to practice tactics. The site has a nice interface and is very user friendly. ChessTempo also offers endgame training as well and has very large database of games, well over 2 million games. If want to pay for membership ChessTempo has some very useful features, but here we're concerned with FREE. You have a choice of solving problems in blitz (timed) or standard (untimed) modes. Naturally, the more you're willing to pay the better the benefits, but the free material is still good.
And, if you're like me and are no longer interested in studying to improve your game, but just like to mess around solving puzzles (or just see if you've still got the old tactical mojo) then the free mode is an enjoyable way to pass the time.
The one disadvantage is, of course, nothing is explained, but then again, maybe that's not a bad thing. When you're sitting at the board you don't get any hints and if you're serious about improving, then sometimes the best thing you can do is figure things out for yourself.