Redis was selected over MongoDB and Cassandra since it scales down, is lightweight (with virtual memory enabled memory is not an issue anymore), and extremely easy to install and manage.
I'd say both gave me exactly what I needed at different times, and so I recommend both projects highly, depending on what you need. CouchDB is an excellent tool to quickly "mock up" a live back end, and a great way to learn hands on with document stores with very little upfront effort required. MongoDB seems to integrate better as part of a larger system, and provides more flexibility.
Because the #1 goal is reliability, we've hesitated to recommend production use, until the most recent release (0.11). CouchDB 1.0 ships this summer, and is definitely a safe place to put your data.
No background process like mysql or pgsql. Thus helps me run on cheap ram. _IF_ it grows and gains traction, then switch to a better db, which until now has not happened. So I haven't thought about the "better db" i mentioned above.
However, RF does about 80,000 pageviews per month, has over 1000 registered users and gets about 10 posts a day, and SQLite hasn't proven a problem whatsoever. It's hardly a big project but it's bigger than 90% of small, part-time projects that use technologies that are more advanced than they really need.
You must mean Derby..
another cool area might be analysis of data structures to predictably suggest better schemas for faster results, or better database engines, for example.
In my experience, MySQL and PostgresSQL are both awesome choices, and the community support behind them is huge.
Looked at using it for an app I've been hacking on. Getting it to market seemed faster to do on a traditional rdbms, but I may migrate later once I incorporate collaborative filtering. Just curious if anyone has significant experience with it :)
Playing around with the document/NoSQL databases, but nothing in production yet.
It's a proof-of-concept other people will find useful when I get around to make it presentable.
I wonder how many in the "Other" category are SQLite as well.
MySQL, PostgresSQL, SQLite, and SimpleDB.