Just a quick note as I know there are a lot of users here of the free WP theme, Flexibility -
it's been recently updated to Flexibility2 - which if you haven't updated the original, this one gives you the ability to change and manage everything from your dashboard, including full color change, headers, content box and sidebar width, etc. You can add widgets under the header and in the footer as well, so really easy to add affiliate programs etc. You can also change backgrounds which is neat, and can give your blog a nice 'custom' look, too.
You can also add Adsense to your posts from the admin.
It also has a new paid upgrade, which creates a really neat sales page with WP but you can still get the free theme at Download Flexibility Theme for WordPress (not an affiliate link!) - it's a bit tougher to find now that they have a 'premium' version too -
Just to give you an idea of the different looks you can easily create, here's some blogs I did last week, with basically the same layout - just changing colors, headers, background:
Fast Insomnia Relief
Grants For School or Business
Needless to say - I love the theme!