The wonderful Victoria of Writer Beware chose to point out another scam today - so I'm going to signal boost.
Agent middleman services. They promise to make sure your query gets read by an agent. Many of them will put you together with a professional editor (Hint: Although professional editors are vital if you intend to self publish, their use is a red flag if you send a manuscript in to an agent or publisher - the concern is that you don't know how to do the necessary self-editing).
Then they'll charge you a nice fee to...do what you should be doing yourself. Finding the right agent and querying them. Of course, can you really trust these people not to send your manuscript to an agent who doesn't take your genre or, worse, a fly-by-night, fee-paying, or fake agent. Likely a fee-paying one if they do "find" you an agent. These people tend to scratch each other's backs.
Don't do it.
If you want your query letter critiqued, try an online writer's forum or group - there are quite a few out there where people who have already been published will gladly tell you what works and what doesn't.
If you want a quick way to find the right agent, try agentquery.com or another database site. They're free, and they list agents by the genres they take and often keep on track of who is open to subs and who isn't.
These services are at best unnecessary, at worst scammy...and many reputable agents block their emails when they come across them.