TwoDimes.net:
This site allows you to calculate probabilities for various poker situations.

Neil Dewhurst's Hand Converter:
Converts boring hand histories into elegant well formatted summaries of play.

Poker X Factor:
Converts hand histories into animated re-enactements.

Poker Tracker:
Downloadable software that tracks online player statistics.

Shark Scope:
Allows you to query Sit n Go statistics for online players.

Tournament Reporter:
Like sharkscope but for regular tournaments.

Poker Stove:
Downloadable software that computes various poker scenarios. Similar to two dimes, but much more advanced.