Earlier in this blog (G-d I hate that word) I stated that it's okay to email queries to agents if you the address from a major website. Miss Snark pointed out to me that it's fairly hard to tell what's a major website or a reliable source of material (See my post on Writer's Digest).
I would like to amend my comment and say that you should email a query after checking an agent's website. Also, AgentQuery is very respectable and the agent will list on their profile if they are accepting email queries. Ignore other information.
My apologies to the other agents and assistants out there.