TinEye - will search for an uploaded image anywhere on the web (good for finding out if someone’s poached yer stuff).
Tynt - inserts code into text that is copied and pasted from your blog (for those non-Tumblr blogs that don’t just hit the “re-blog” button). It can be disabled, but it inserts attribution code with the text.
Someone out there on teh interwebs apparently wrote 14 chapters (plus a prologue) on the theology of Calvin and Hobbes. I’m not sure I have the time or interest to read all of it, but it’s rather amazing it exists at all.