How can I learn combinations without a chess book? I would like to become better and stronger at spotting combinations. I can't buy any chess books at the moment. What's a good website for learning combinations?
Chesstempo has a tactics trainer with solving against the clock and a tactics rating. I think there are a bunch of other sites out there with similar features.

If you are rather old school WTHarvey has 10,000 chess puzzles, just as a collection of diagrams with mouse over solutions.

Chessgym also has tactics against the clock and some other features like playing endgame positions against tablebases.