Low disk storage Space
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 -&gt; Settings -&gt; Applications -&gt; Manage applications -&gt; Menu -&gt; Sort by size.  Then tap on the largest apps and tap on the Clear Cache button.  GMail, Browser and Maps are known space hogs.

You can try various apps that clear cache with one click.  For example, aCCleaner or CacheMate.  Keep in mind that some of these apps require root.