1. Blogger. It's super easy to use, and I didn't realize I could set it up to publish via FTP until I set it up for this site. So now all I have to do to publish a post is login to my blogger account, write a new post, and click "Publish Post", and the HTML files, and RSS feed are all updated on my site. The side benefit is that my posts are also backed up at blogger.com. There are certain limitations of doing it this way, but it's still an impressive service for free.
2. Gmail. I recently learned that Gmail can be setup for an external address. I was able to setup my bucketsoft.com email address in Google Apps, then change my MX records to point to the Gmail server. And then, voila, I get emails through Gmail for free.
3. Google Analytics. Man, years ago I wrote a web traffic tool that would gather statistics and would allow you to view reports and graphs in a similar way. I went through 4 complete rewrites of that code. But when the company I was working for folded, I lost the code (and they never did anything with it). It was pretty cool, and fast... very fast. But I must admit it was never as advanced or flexible as Google Analytics. I've used Google Analytics on every site I've made for the past few years, and of course I'm using it here as well... for free.
4. GetSatisfaction.com For those who aren't familiar with this site, check out my page here: getsatisfaction.com/bucketsoft
This site is extremely well done. My page may not be the best example because it's pretty empty right now. But I love the way their feedback engine automatically shows popular subjects so you don't necessarily have to ask a question if it's the most asked question already. Oh yeah, it's free too.