I've been extremely fortunate to be in some guilds with exceptionally generous people. When I was levelling my first character, a warlock, I found a very nice guild where people shared a lot of drops. I was out working on level 19 one day when a hunter on guild chat mentioned that they'd had a set of Black Velvet Robes drop. For a level 21 caster item, it's pretty much the best thing ever. I said something to the effect of "wow, those are really cool" in guild chat. An hour later, they were in my mailbox. I was just stunned at the generosity. Those robes sell for a lot of gold, even with the level 21 requirement making them less desirable for twinks in battlegrounds.
Is this sort of generosity common? I have a very limited perspective, having been in exactly two guilds, so I'd like to hear your stories. What kind of items have you seen given away (or sold at well below cost)? What kinds of criteria are used for giving things away? I'm less likely to give something to someone's alt than their main, but I'd like to read your comments on this.