Happy Cyclope is a 2 columns, easy-to-customize wordpress theme. It is of course widget ready and WordPress 2.5 ready, but hasn’t been tested with previous versions.
Categories and archives pages are presented as lists, which makes it nice for sites with a lot of content. The header is made of two images, one for the logo and one for the header. To customize your header, just change these two images and eventually change the background color.
This theme is also valid CSS & XHTML transitionnal, as well as free and open source.