Blogopoly

Apart from random advertising, there is another way to make money from your blog using partners: affiliate links. The Ad Networks described in my last post will normally pay for clicks - when a user clicks on the banner you get a few cents. With affiliate links you are paid by your partner when the user makes a purchase. This may sound like a worse idea than regular ads, but you can find some very relevant affiliate links, with advertising you cannot control the content of the ad or product supplied. For example, almost every blog will have some books covering similar subject matter. Amazon will pay you a percentage of what the user purchases when transferred from your blog - you can just put their logo on your site for users to click on but you can also put a direct link to books specific to your subject matter. Similarly you could put links to specific CDs, DVDs, Videos or anything else on their site. If your site mentions any media, then you can put hyperlinks in your posts to their product pages on Amazon too. A good example is this post here on the Booklinker blog.
I have used Amazon as an example, but they are not the only ones, here are some others you can also use, including Amazon:

Amazon Associates Program
Ebay Affiliate Program - offers payments for new registrations and bids
BOL Partnership Programme - note that BOL has no US site.
Barnes & Noble.com Affiliate Network
Buy.com affiliate Connection - US site only
Best Buy affiliate Program - US site only
BlackStar - UK site only
Quelle - Germany site only

Apart from those mentioned as being based in just one market, there may also be programs for your country too, so it's worth checking out the local site. The list above is just a starter and these are retailers of a wide range of media and goods. There are many specific affiliate programs, even one that pays commissions when you refer users to a service that helps retrieve their FBI files! Simply search on Google for affiliate programs and you will find many directories pointing you to more specialized programs.