I'm surprised not to see MongoDB mentioned; it's the document-oriented database I find most intriguing:
I don't know enough about its alternatives to offer meaningful comparisons, but things I really like about MongoDB in the brief time I've played with it:
- It's trivial to get started with.
- Everything just makes sense.
- No schemata to deal with.
- Excellent Ruby support right out of the box.
For the very simple use cases I tend to deal with, I've always defaulted to throwing my data into MySQL or SQLite, and I think MongoDB will replace them for me.