Much of the tuning and theory around maximizing your impact as a raider comes down to simple math. Knowing the mechanics of the game goes a long way to getting the best possible results. This usually consists of a lot of reading, research, testing, and the use of spreadsheets to do the mathematics for you.

Additionally, for DPS classes in particular, you may be interested in Warcrafter for sandboxing your toon's paper doll and making changes to your talent spec, gear, enchantments, gems, and inscriptions. There are a lot of variables to consider, and ample opportunity for being the best you can be. It is a good idea to start there and then use the spreadsheets available to model other factors that the prettier tools don't account for.

These spreadsheets are usually made for Microsoft Excel, but can often be used in Open Office (and the folks that make these spreadsheets are generally receptive to making them work with Open Office) but Apple's Numbers can be very hit-or-miss with them.

Many of these links are going to go to the Elitist Jerks forum. This is because they have a very high signal-to-noise ratio and the people that post there aren't your typical dumbass. If you end up going to EJ and start using their forums, please be aware of the forum rules and don't do stupid things because they have a very low threshold for people that can't use Search or read a discussion. Usually in a compendium post the first post will be "the bible" and the discussion that follows may or may not result in updates. For the basics on applied theory in WoW, check out the EJ Theorycrafting Think-tank and WoWwiki also has a lot of good information.

Tanking

Druid Tanking Program: Rawr (Rawr now also has many other models for Paladins, Druids, Mages, and other classes.)

Of course, Paladin Tanking is well covered.

Healing

Restoration Shaman should check out How to heal like a pro @EJ.

Holy Priest? Healing Priests and Raiding @EJ is fantastic.

Holy Paladin guide for WotLK http://elitistjerks.com/f76/t35975-holy_paladin_guide_wotlk/.

DPS

Rogue DPS (and gear) Spreadsheet: @EJ

Shaman: Enhancement DPS Discussion: @EJ

Feral Druid DPS Spreadsheet: @EJ
(May need to dig a little to get to the spreadsheets. Bottom of Post #2, Volden's is excellent)

Hunter Spreadsheet and other information: @EJ and some great information on survival hunters post-2.1 patch. Every raid should have one.

Mage DPS Spreadsheet: Needs update

Shadow Priest DPS Spreadsheet: @Shadow Priest.com

Elemental Shaman DPS Tool (and some Discussion @EJ)

Lots of other Shaman stuff

Warlock DPS Spreadsheet: @EJ has been picked up again and is now maintained by someone else, but still hosted in the same place. This discussion thread will help you get started.

The "Warlock PVE Compendium" is gone, but until it is revised and updated for WotLK, there are some good discussions to follow. Until it's done, the PVE Discussion is good, though not complete. You should also search in the Simple Questions and Simple Answers thread.

Warrior DPS Spreadsheet: @EJ

Retribution Paladin DPS: Retribution Paladin Thread