I've written about this before, but seeing as how it is a particular pet peeve of mine, and how the internets still displays a need to learn this, I am mentioning it again.
There is a difference between the words who and whom. Namely,
Who is used to replace subject pronouns in sentences (I, we, you, he, she, it, they)
Whom is used to replace object pronouns in sentences (me, us, you, him, her, it, them)I like to use the him/them test, since they both share the ending -m with whom. If you are trying to decide between who and whom, and you can grammatically replace your who/whom with him or them, then whom is appropriate. If you can't, then it isn't.
You don't say "I went to visit my friends, whom are very nice."
No! You don't say that! If you do say that, maybe you are confused because the who/whom of the second clause refers back to "friends" in the first clause, which is objective. You could replace "friends" with "them". I went to visit them. But the who/whom in question is in its own clause, and in that clause it takes on a subjective role. You wouldn't say "Them are very nice," but "They are very nice." So who is the appropriate choice.
Please take note of this very helpful test and use it. Or else I will come through the internets and beat you over the head with a shovel.
When in doubt, just use who, alright? It's ok to be wrong with who. Whom is kind of antiquated anyway, and I predict that one day it will leave our language altogether. But when you are wrong about whom, you just look like a wanker who is trying to be proper but doesn't know what the hell she's doing. Mkay?
In other news, for all you fellow voyeurs out there, today a Blog Share event is going on, where bloggers post anonymously on other blogs so that they can vent about something they'd never post on their own blogs under their own names. These posts are usually very juicy. Go here for a list of participating blogs.