Gotta root to really effectively solve that error. 1Tapcleaner helps, but you need to see what's taking up space in your data/data folder on the sd card, and you can only check that if rooted - DiskUsage is the best app for that. That folder only holds 150 mb of data, lib, etc pieces of apps and when it gets to less than 10mb or so, it'll trigger that error. And it fills up fast as you use some of your apps (like browsers which pile up cache space, etc).
Opera browser, Touchdown, Contacts all are big offenders with this folder.
I used to get that low on space error all the time and it drove me insane (and made my phone sluggish and stopped apps and email from synching til it was cleared). Rooting helped a bunch and now i can monitor that data/data folder so I don't get anywhere near that error getting triggered. Rooting is really the way to go!