I pretty much use this app for one thing only, but that one thing is so utterly indespensible that it'd be worth money if it weren't generously free:

Converting non-English text to HTML entities via the services menu.

When inserting small blocks of Japanese characters into English web pages, being able to select the text in TextWrangler and just select convert from the Services menu has saved me so much time I don't even want to think about it. It even provides a nice little transparent overlay telling you what it's just done.

The wide variety of other features I've poked at on occasion, and the spotlight integration is cool, but basically this app provides the most valuable Service on my computer, and has never caused an issue or not functioned as expected (even version 1.6 at the time Tiger shipped worked perfectly with it).