Well there are two links that showcase 11 and 10 fixes and hacks respectively.
Most Desired WordPress Hacks: 11 Common Requests and Fixes: Noupe lists 11 commonly requested WordPress hacks and elegant fixes for them. They include avoiding duplicate content, having category specific menus, sidebar login boxes, most wanted categories etc.
10 WordPress Hacks to make Your Life Easy: Jai lists another 10 hacks to play with. His hack tutorials include adding gravatars to comments, Twitter, image gallery, author bio etc.
Some of these are just tutorials on how features work within WordPress and how to incorporate them into existing themes while others are all our modifications of code in themes and in various other places. None of them look too difficult and some of them can be accomplished with plugins. However, all of them are worth checking out.