One of the best resources I've found in "Genealogy Land" is the Message Board. I have benefited greatly from the friendships and "acquaintanceships" (is that even a real word?) made from the various message boards that I frequent. People have helped me fill in gaps, and been generous enough to share e-mailed copies of photographs, marriage certificates, letters and even their own research.
Of course, as my list of contacts grows, so did my need of finding a way to keep track of these wonderful people! They started off just as additions in my e-mail address book, but that just didn't give me enough information for the long haul.
What I've done is create a spreadsheet that I call my "Genealogy E-Mail File" (catchy, isn't it?). It's very simple: just four columns: Name, E-Mail Address, Family Line, and Comments. If I were corresponding with these people via "snail mail," I'd probably add a mailing address column, too -- but in general, my contact with these researchers is strictly via e-mail.
The "Family Line" heading is the surname that we have in common. Under "Comments" I make any notes that will help me in the future. Most often that is the name of the ancestor that they descend from, but sometimes it's the fact that we haven't found a connection yet, but I've promised to contact them if I do find a common thread.
I'm sure that most genealogists have some type of similar system -- or possibly a much better one -- but this one has worked for me so far. The camaraderie within the genealogy circle is one of the best parts of this business. I'm so grateful for what these nice people have shared with me -- if/when I break through one of our common brick walls, I sure want to be able to find them to share the news!