Seriously, people.
Yesterday Nathan Bransford shared his query stats from the past week. I'm dismayed by some of the stats:
9 mass mail queries (often with every agent's name visible)
2 misspelled his name
1 addressed to Mr. Brown (Nathan works at Curtis Brown, Ltd.)
2 angry replies to his rejection
23 personalized
11 began with a rhetorical question.

OK. I'll over look the rhetorical question starters. I'm sure the authors really, really believed that was the best way. Or they read a book that told them it was good. I'll even pass by the misspellings. It happens. Although I will say, check the name, then check it again. The two angry replies? Get over yourselves. This is a business and if you can't take rejection, you really don't want to be a writer.
But the rest? That is just laziness. Someone actually addressed the letter to Mr. Brown. I'm pretty sure they meant Mr. Albert Curtis Brown who passed over to the whatever a long, long time ago.
The nine who sent out mass mailings--I don't know what to say. Do you really believe that's a good way to contact someone that you are asking to read your work and possibly represent you? Do you think that impresses them or makes them feel they are dealing with a considerate professional writer? Um, not so much.
The 23 non-personalized. I read Nathan's blog about this and I know he's talking about personalization beyond "Dear Mr. Bransford". I've read about agents who really prefer this (Nathan, for example), and agents who really don't give a rat's ass one way or another. The thing is, if it's done well, it can't hurt and could possibly help a lot. But don't be smarmy or stupid. If you're going to personalize say something about their clients, their blog, a recent sale, the fact that you met them at a conference or heard them speak somewhere. This requires research, but really not that much.
If you are looking for an agent, Google (or the search engine of your choice) is your friend. Use it.