Here's one: PortaBase. The beta versions have been built for the N900 and available for a few weeks, but couldn't be installed easily because of the Qt package transition. With PR1.2 out, I just finally managed to install it on my own device (I'd been using it on my N810 for the last month or so). I just released a new version that fixes all the serious bugs reported on other platforms so far.

The "exciting" part? It already works on the N900, N810, Windows, Linux/UNIX, and Mac OS X. And it shouldn't be too hard to port it to Symbian, Windows Mobile, and maybe even webOS. There's even an actively maintained port of Qt to OS/2, of all things...

So if you want a data management application, give it a try and let me know what you think. It's currently in extras-devel, I'll start moving it into extras-testing and extras once I get some more feedback that it's working ok for people (or fix any new bugs that turn up). There's another thread already discussing it.