Idea Chess offers: Checkmates in 1-4 moves. Tactics from easy to difficult. Endgames. 100,000 interactive chess tactics, strategy and mate problems. One very nice feature is that if you are wrong, you can play it out against an engine so you can see why your solution doesn't work. Another really nice feature you can export the FEN of tactical puzzles. If you register (FREE) you can use the site's rating system to track your progress. The interface is nice and clear. They have a forum, but there isn't much activity on it. Excellent!