Cassettes & Freewheels
If you're thinking about changing your cassette yourself keep in mind more than likely your chain should be replaced as well. You'll need a couple of specialized tools.
Shimano XT M771 10-Speed 11-36t Cassette
SRAM PG-850 8-Speed Cassette
SRAM PG-950 9-Speed Cassette
SRAM PG-980 9-Speed Cassette
SRAM PG-990 9-Speed Cassette
Shimano HG50 9 Speed 12-25t Cassette