Everyone who has a blog needs readers. To gain readers, many bloggers choose to advertise. That may sound like a very mercenary way to draw readers to a personal venue, but one has to get the word out by some means.
Get traffic to your blog:
There are three major blog advertising networks: Entrecard, CMF Ads, and Adgitize. All of these will increase your bounce rate, if you care, but a blogger has to do something to get people to see their blog. The idea is that these methods will get people to your blog. It's up to you to provide content that makes them want to come back. I have several blogs, and use all three of the networks to advertise.
Brief thoughts from my least to most favorite:
Entrecard (EC) has had a lot of serious ups and downs and angry clients because they keep yanking our chains with unexpected changes, but they are still free and will bring in traffic. You buy ads with credits that are generated through clicking on other blogs in the system. You also receive a portion of the credits paid by others to advertise on your blog. You will have to do some clicking (called dropping) to earn enough to buy decent ads...so this is a disadvantage for someone with little free time. But they are huge, and have a big cross section of types of blogs. You can also pay cash to buy ads, but there is a lot of negative backlash from when they added this and a lot of people will reject paid ads out of hand. You can sign up at Entrecard
My next favorite is CMF ads. You can also start there with no money at all. When you get a few ads on your blog, at the end of 30 days you will be paid, and you can place ads on other blogs in the system. They just (Feb 2010) started ads across the whole network for $10 a month. That's still in the process of being sorted out. This is a smaller network, so you don't get as much exposure, but it's purer- there is less monitoring of blogs on Entrecard. You can also choose to put some money in your account and start buying ads and "spikes" right away, so that people will begin to see your blog. CMF will generate the least traffic, but it also takes almost no work on your part. Sign up at CMF Ads
My hands down favorite is Adgitize. If you have time to click on 50 blogs a day you can earn back a little bit more than what you pay for an ad, making a small profit. You pay $14 a month for an ad in the system, but because of the click system this will practically guarantee you about 70-100 hits a day. If it's worth the $14 you can just buy an ad, and not worry about getting it back. You could choose to be just a publisher, and you would earn a few cents a day, but to get other people to come to your blog you need to buy an ad. Adgitize doesn't pull people's chains, change the rules without warning or treat people poorly (EC has a terrible reputation for all of the above). Adgitize planned a business model that works, and doesn't have to tinker with it and drive people crazy to try out new ideas. You can sign up at Adgitize
For a much more complete coverage of the three networks see my article Comparing Entrecard, CMF and Adgitize for Blog Advertising