You'll be amazed at how the most simple things sometimes work the best. For instance, I just put a poll on my blog (not the one in my sig) and sent it out to my list this morning. Under that poll I put a ClickBank link to a product that is directly connected to the subject of the poll. In the first 30 minutes I have had over 200 people participate in the poll, with 12 clicks on the link. By the end of the day I expect to have made several sales and didn't spend a dime.
P.S. Polls are old school, but they are very effective. People love to let you know what they think on any given subject and it is a quick, anonymous way to do it.
P.S.S. I also sent the poll to my free Yahoo group <---another old, yet effective source of traffic that costs nothing.