I disagree with Alfiet. Particularly at the moment. With so many CV's it is good to have something that makes yours stand out. The cover letter does not have to be a separate document. It can be the body of the email (or further information section on most job boards).
If applying for a particular job:-
Sell yourself to the recruiter. They have to sell you, so give them all the ammunition you can. Your CV should start with a profile that is specific to the job you have applied for. Do Not say "I work well on my own or in a team, I am hard working, I have great experience in this field etc" Back it up. I worked as part of a team in company x where we achieved y. My referees will attest to my work ethic. My personal responsibilities were x, y, z where my self-motivation shone through enabling me to reach/exceed all my targets.
Make is fairly short. You want to tell the recruiter why you are the best person for that specific job. What are your unique selling points?
If sending your CV in on spec:-
Again a short summary, explaining, who you are, where you have come from, what you are looking for and what makes you the right person for that type of work.
We are seeing a huge increase in the numbers of people who are applying for jobs that bear no correlation to their CV. You have to show a strong link between your experiences and any job you are applying for.
It depends on the Agency, but some may not want to meet you and waste your time and theirs if you do not fit the bill. If you wish to meet them however, and would like advice from them, then there should be no agency in the country that will not!
If your CV is rejected by an agency for a specific role, it is not the end of the road. Ask them for advice. There is always something you can do to improve your chances. Stay in contact with the recruiter. Don't let them forget you. That way, when a suitable job does come in, you will be one of the first people they think of.
The relationship with a recruiter is a funny one. On one hand, you should be selling yourself to them, but on the other hand, you need to get advice from them too. They know the client companies, and what they look for. (if they don't get a new recruiter!).