http://www.agentresearch.com
http://www.anotherealm.com/prededitors/
First, visit those two websites and bookmark them. Second, any agent you are interested in, check with with both sites (agent research has a free agent verification service, just click on "our services" and order the free agent verification--it's online and quick)
Because there are no real regulations on agents, the gammet ranges from snake-oil salesmen to legitimite agents. The rule of thumb is, if they ask for a single dime BEFORE you sign a contract with a royalty publisher, then they're not a real agent. Real agents will charge commision (usually 12-15%) and office expenses, but these come out of your royalties. A not-so-real agent may try to coax you into paying up front expenses, and some are nothing more than feeders for vanity/subsidy publishers.
In most cases, an anget will get your manuscript read by the editors who are most likely to buy it quicker and more effectively than you can; however, if you query publishers who are open to submissions yourself, you might get a standard contract (in other words, the same royalties that you would have gotten with an agent) without having to pay an agent's fees or commission.
Personally, I have no experience with agents, because all the agents I've queried have shown absolutely no interest in my novel

But, The above represents what I've learned from doing my homework before sending (and I still have 7 good agents that I haven't queried yet, so all hope is not yet lost).