In-Text-ads

You have must have seen them in lots of places in the net. These are the double underlined words which show a pop-up when you place your mouse pointer on them. These are called so because they are blended within text and are not stand out advertisements.

Kontera

They are one of the early pioneers of this model and are quite famous.
You can sign up here.

Scribe Fire

This is one of the very popular ad networks. They also offer various forms of ads like banner and quick ads. Quick ads give you the ability to click the desired ad and move it to your blog immediately.
You can sign up here.