If you're looking for a lot of bang for your online advertising buck, you should consider trying out MetaFilter's TextAds. Ads are priced at $2 per thousand impressions, with a minimum of 5000 impressions. MetaFilter, on the whole, gets a fair amount of what I would call discerning traffic, people who are on the lookout for new and interesting things (or something like that).
One possible use for TextAds: drop a measly $10 (for 5000 impressions) on an ad featuring your personal Web site or weblog. Getting readership for personal Web sites can be difficult; TextAds might be a quick and easy way to jumpstart the process.