1. Google ads
Register at Google Adsense and start showing related advertisement on your blog. You can also use Feedburner (Fedburner>Feeds>Optimize>Feedflare) function and paste the code in single.php and index.php files in your template to show ads in your post itself. There are a limit of 3 ad units per page. A combination of text/image ads, google custom search engine and link ads is always works good.
2. 125×125 Ad spots
You can have more than 6 items in this type of ads. There are so many free wordpress plugin that helps you to manage these ads.
3. Text link ads
You can use Scratchback (known for its low fees and following google’s rules in text link ads by passing nofollow links). You can also, sell it on your own by adding a few text and hyperlinked codes using Text widget in your wordpress. You can use some free html editors to edit this links too.
4. Paid post networks
Paid post is one of the best blog income that anyone can get. Make sure your blog is at least 3 months old with an average of one post posted in 2 to 3 days (min 10 posts each month). If not, try to use timestamp and adjust it. There are more than 20 networks are providing this services.
5. Header / Sidebar / In Content / Footer Custom Banners
The best way to let your visitor know is by having the “Advertise here” badge on all empty banners. Or, you can have a tiny link with rate as label below banners.
If you have more idea, please do share.