I tried Google's AdWords
which cost $15 for 1,000 impressions. The click-rate was about 3%-4%, (about 38-50 cents per click). This is not too bad, but if you can get top ranking on google that's much better.
(used to be goto.com) is also very good, especially if you don't have too much competition for the word you bid on.
Yahoo is usually worth the money, if your site is making a profit. I got listed for free
DMOZ is free, so you should get in their directory if your site is good.
The only way to get into altavista is if you pay them, $30 for the first URL and the rest are about $20. They are getting much less traffic lately.