Have you ever wondered of getting paid for writing posts? , isn't this cool. Its true, you get paid for writing posts. The amount they pay you differs. Some site requires your blog to be few months old also with some average posts every week.
what you need to do is write reviews about some products/services ie. you are indirectly promoting the advertisers content (the reason to get paid).

The advertisers will choose you only if you have lots of visitors and importantly a good page rank. Some sites also require you to have your own webspace hosting your own blog (wordpress) so using free blogs goes out of the list there.
These oppurtunities fetch you a lot.

You are provided with vast topics on which you can write posts. So its upto you to choose a right topic, on which you feel ur capable of writing a post or a review.

Here are some top paid2blog sites :

PayPerPost
Payu2blog
Blogitive
Blogvertise
LoudLaunch
Bloggingads
BloggerWave
V7n
Money4blogs
Sponsoredreviews
LinkWorth
Blogtoprofit