For the last couple of months my Motorola Droid running Android 2.2.2 has been complaining about being "low on space" for the phone, not the SD card. I pruned some apps, but that didn't help much. Things really came to a head this morning when my phone was so low on memory that it was no longer downloading email.
I found this article to be quite helpful -
For me, the two big ones were Messaging and the Browser cache. I had a couple of threads in Messaging containing a number of pictures. Once I saved the pictures off to the SD card, I purged the threads, that freed up ~20MB. Clearing the browser cache freed another ~20MB, but that will probably evaporate again as the browser caches things.
Here's a minor whine about Android: the SD card and phone storage settings page tells you how big your SD card is and how much space is remaining, but the phone storage just says how much is left. Without knowing how much I had to start with, it's hard to know if, say, 20MB is a lot or not. As near as I can tell, Android seems to complain when the space is less than 25MB.
Update: I ran out of space again, and clearing the browser cache didn't help. After bumping around some more, first I discovered that in "Manage Applications" the one and only menu option is to sort by size. Doing that revealed that the new pig was the (post pay-wall version) New York Times application. It was using over 60MB of data space in the Phone Storage area. The app doesn't have a "clear cache" function, so I used the "Clear Data" button from within Manage Applications, and I was back in action.