My knowledge of slash commands is by no means extensive, but it always amazes me how much I can do without touching my mouse. Here are a few:
Will try to keep this updated as new commands surface.
Normal slash commands:
/cast (spell) : casts the spell. Useful for macros, somewhat useful if you're a fast typer for weird stuff and don't feel like opening your spellbook. /use is similar, but with items.
Example:/cast will of the forsaken isn't fast, but is awesome
/m : opens up the macro pane
/gm : opens up the ticket pane. This is super useful for me as I have the help button hidden
/camp: logs you out of the game. Especially useful for cancelling flight paths; just log out then log back in.
/rw (text): flashes the text midscreen for all raid members, if you're a raid assistant or leader, or all party members normally. Only bears mentioning because you can do it in party if you're not leader.
Ace only normal slash commands:
These work only if you have a mod that uses the ace library installed.
/rl : reloads your ui. The same as /console reloadui or /script ReloadUI(); but a bit faster to type.
/in (number) (command): useful for macros mostly; fires a non-combat event after (number) seconds.
Example: if you wanted to /say "sheep ending!" after ten seconds, you could do
/in 10 /say Sheep Ending!
/print (expression) : evaluates an expression. Essentially, this lets you do math in-game without tabbing out to a calculator (I use it for things like summing avoidance, estimating profits, adding mat costs, etc). I tend to use things like (10 + 3) / 5 as my expressions, but I'm pretty sure you can use any variable or operator you can access with the api rather than just basic math functions, if you swing that way.
Example: /print 10 +3 returns 13. /print 10 /3 returns 3.333333333
Console Commands
These don't require any mods to work and aren't case sensitive.
/console reloadui: reloads the user interface. If any part of your interface bugs, this command often fixes it. Similar to logging in and out for many purposes, but faster.
/console gxrestart: reloads the graphics engine. Very similar to reloadui, but with the 3d world rather than your 2d interface. Useful after you get the bug where everyone's naked.
/console cameraDistanceMax 50 : enables you to zoom your camera out a wee bit farther.
/console cameraDistanceMaxFactor 4 : enables you to zoom your camera out a fair bit farther. If you're curious, 3.4 is the real max rather than 4, but 4 is easier to type.
Script Commands
These also don't require mods but ARE case sensitive.
/script LeaveParty() : removes you from your current group. Useful when you get those weird bugs where the game thinks you're in a group but you can't remove yourself via your player frame.
/script RepopMe() : useful when you fall through the ground (GOOD FUCKING GAME BLIZZARD), normally resses you.
(shamelessly stolen from Lulzmachine in the comments below)
/script ConfirmSummon() : accepts a summon, for when the raid leader swears he's given you three.
/script AcceptResurrect() : this command lets you accept a res from a player when the box fails to appear.
/script RetrieveCorpse() : as above, but when running back in ghost form.
/script RepopMe()
Useful when you fall through the ground (GOOD FUCKING GAME BLIZZARD), normally resses you if you've been a good boy.
/script px,py=GetPlayerMapPosition("player")
/script DEFAULT_CHAT_FRAME:AddMessage(format("[ %s ] %i , %i",GetZoneText(),px*100,py*100));
Your current location will be printed in the chat box (chat frame) when you use the macro.
NOTE: You may need to open and close your map after using a flight path for the script to give your correct location.