I have not added applications to my phone. I have actually been deleting applications from my phone because it will tell me I have low disk storage space. Sometimes it goes away, but will come back at another time when I still have not added any applications from the market. I'm not sure how to keep it from telling me this...
Cache from installed apps also resides on the phone's internal memory, and if not cleared in a while can grow large enough to fill it up. Go to
Menu -> Settings -> Applications -> Manage applications -> Menu -> Sort by size. Then tap on the largest apps and tap on the
Clear Cache button. GMail, Browser and Maps are known space hogs.