RsWebParts made the Cool SharePoint Stuff list on Greg Hughes blog. There is some very cool stuff on that list that I'm hoping to have time to dig into myself. Here are some cool things from that list plus a bunch of my own:
Wow. That should keep you busy for awhile! I'm sure there is more that and this doesn't include all kinds of interesting SharePoint discussions such as Sharing your bookmarks: http://del.icio.us, wikis, and the zen of SharePoint. Enjoy!