NetTuts has put up a list post of fifty of what they consider the top WordPress tutorials. It includes some very helpful posts, including a few from their own blog that are definitely worth reading.
I found some to be more useful than others. Some tutorials are just learning one or two line PHP scripts that will help you add or remove features from your blog, while others are thousands of words long, and require almost a weekend course to understand.
If you are looking to modify your WordPress installation in new and interesting ways, it is worth checking out this tutorial list.