Well, WoW, Everquest, Everquest II, Guild Wars, Ultima Online…I’d say that most fantasy-based MMORPGs use “guild”. Other genres tend to use other names (“supergroups” in CoX, “corporations” in EVE Online, “linkshells” in FFXI).
That too. 
Alternately, to cover the cleavage shown by some armor.
My guild leaders can be counted on to redo the tabard to show something obnoxious for April Fool’s. One year it was hot pink and had a big Greek omega letter on it (leader’s character’s name is Omegamoo); this year I think it was changed to a bunny or something similar.
jayjay: Agreed, and most MMORPGs seem to have a decent measure of in-game support/functionality for guilds too.
And RIFT, which I’ve just started playing, is also guild-based with special guild achievements.
Our guild leader auctioned off our tabard design in order to build up the guild bank coffers one time - it was scary seeing as some of our richest members were children. But it all worked out okay.
As for cleavage - it was only scary on the men 
Women could always stick a shirt on underneath.
But some of those robes were scary on the men - just scary!!
The same phenomenon occurs even if they’re not directly supported by the game, though. I think all modern games support guilds or the equivalent, but some older ones (Diablo II comes to mind) didn’t.
It should be noted that guilds aren’t necessarily strictly private concerns…there are certain guilds in WoW, especially, that are relatively famous within the game’s sphere of interest. Ensidia/Nihilum, Paragon, Death and Taxes (at least back in Vanilla WoW), and of course, Leeeeerooooy Jenkins’ guild, PALS FOR LIFE (even if the video was staged).
My own guild, Wreck List, on US-Garrosh, is making a bit of a name for itself. They were server-third to reach Guild Level 25, and our progression (which I’ll admit I don’t pay a lot of attention to, as I don’t raid) seems on the high end. I don’t know how “famous” we are, though.
Oh, and there was an EVE Online corporation that made news a couple years ago for completely taking over another corporation’s finances. Virtual reality corporate raiding! Unfortunately, I don’t remember the name of the corp, but they get a pass on that, since I’ve never played EVE Online. If I did, I’m pretty sure I’d have remembered. Like I noted above, “in their sphere of interest”, and EVE is about as far from WoW as you can get and still be an MMORPG. As someone once called it, it’s “the prettiest economic simulation ever invented”. EVE players are serious math/statistics/economics geeks.
Also, orgies.
Endless orgies.
Only in the Tram tunnel…
Ah yes, my Guild Leader changes around the rank names in a similar fashion… often with a seasonal theme. At Easter I think I was a “Chocolate Bunny”.
(She used to run a hard-core raiding Guild once upon a time and never wants to do that again – keeping the ranks silly and de-emphasising the structure is a nod towards the casual nature of the Guild).
Regarding meeting or knowing people IRL – our Guild started with people who all knew each other IRL (having been trad pen & paper RPGers). For a long time the recruiting rule was knowing someone IRL, or friends of existing guildmates, but it’s relaxed a little at times, and we’ve had Guild BBQ now and then where I’ve got to meet some of the guildies I didn’t know.
LOTRO calls them “kinships”, but a fair number of players still call them “guilds” out of habit.