This app is great if you're rooted. https://market.android.com/details?id=com.aac.cachemate.demo... Otherwise, I find AppBrain great to be able to easily see space usage and move stuff to SD. I have a Samsung Galaxy Spica (not S, a european phone), and the limit is at about 15 MB when I get warnings. That's hundreds of SMSes... Then again, the Gmail cache eats MBs like a dog eats dogfood: easily 3MB.