Two Column Themes
K2 is a popular WordPress theme which is not fully compatible with WordPress 2.5. The theme now has options to dynamically reduce the number of columns to fit screen size and also support for Hooks and PHP. The theme also has Avatar support and Image gallery templates for WordPress 2.5 and above.
Daydreams is a 2 column minimalist theme for WordPress. The theme has not widget support.
Three Column Themes
Starscape is a three column widget-ready theme which comes in three different colors, the theme also has a extended sidebar in the footer. You can turn the sidebar on or off from the Admin section.
Choice is a simple three column theme which has a configuration file in the back-end that gives you a nice selection of options that control color choice, column amount, number of sidebars and more among other things.
Aqua Marina a three column theme made up of blue colors, the content area is surrounded by two sidebars on either side. The theme also has a area on the top of the page which you can use to display latest posts, recent comments etc.