A compilation of all the macros I currently use, I split out the class specific macros to the following posts:
Druid Specific Macros
Shaman Specific Macros
Death Knight Specific Macros
Hunter Specific Macros
Rogue Specific Macros
Here is a listing of all my generic macros that aren't specific to any class in particular.
I get a chuckle out of combining the items supplied to me by two adversarial classes into a single macro. That and it makes sense, mage food is only useful out of combat and healthstones are only really useful in combat. I included the pre-85 mage food as well for leveling alts with mages that don't quite have the best food yet. You could also add some vendor food/drink to fall back on the same way. I have a separate action bar slots for my vendor food and drink currently, but I may make macros for them eventually.
/use [combat] Healthstone, Conjured Mana Cake
/use [nocombat] Conjured Mana Strudel
Show/Hide Helmet or Cloak
I like to play around with my helmet or cloak sometimes and occasionally pull one of these macros up onto my action bars. Just toggles your helmet/cloak from showing to not showing or vice versa.
/run ShowHelm(not ShowingHelm())
/run ShowCloak(not ShowingCloak())
Sometimes I just can't help myself and mark even when I'm not tanking. I have a small macro to stick a skull on my current target when I left click and a cross when I right click or use a modifier.
Alternatively you could use modifiers to make every raid mark available in a single button, but I'm still trying to decide if such a thing is useful.
Few toons, duals specs on some, and multiple sets of gear on all leads to a lot of different trinkets. I have a couple macros to use my trinkets and show their cooldowns right on my action bars. Also serves as a nice sanity check to make sure you have the right gear equipped.
The most recent incarnation of my mount macro includes combining it with another ability that is only used in combat. This serves three purposes: it cleans up my action bars, I can't accidentally hit my combat only ability out of combat, and I get a speedy hotkey to mount.
The ability I've chosen for all of my toons is a crowd control and/or snare removal ability. For druids this would be "power shifting" or shifting out of and back into your current form, expending a global cooldown in the process. Obviously shifting forms isn't something that burns a cooldown out of combat, but I'm just being consistent between toons. I've also got my mount combined with Cloak of Shadows (Rogue), Tremor Totem (Shaman), Lichborne (Death Knight), Fear Ward (Priest), Master's Call (Hunter), Escape Artist (Gnome), and Hand of Freedom (Paladin).
Right now everyone has 3-4 mounts they use on a regular basis: an Abyssal Seahorse, a flying mount, a ground mount, and perhaps an additional RP type mount. I know my Worgen have use an additional ground mount just for fun when they don't want to use Running Wild. So keeping all of these things in mind my mount macros look something like the following:
/use [combat] Cloak of Shadows; [btn:2] Black Stallion; [swimming,mod] Abyssal Seahorse; [flyable,nomod] Magnificent Flying Carpet; Running Wild
/use [nocombat,swimming,mod] Abyssal Seahorse; [nocombat,flyable,nomod] Swift Flight Form; [nocombat] Swift Zulian Panther; [form:1] !Bear Form; [form:2] !Aquatic Form; [form:3] !Cat Form; [form:5] !Moonkin Form; !Travel Form
As you can see out of combat the Abyssal Seahorse will only be used if you are swimming and using a modifier key, otherwise if you can fly it will use your flying mount and otherwise your ground mount. In the case of my Worgen I use a right click to use an alternative ground mount. Using a modifier will also use your ground mount over your flying mount, sometimes it comes in handy. In combat these macros behave just like the ability or abilities you combined it with.