Kind of hard to explain but lets say you charge your N900 to full battery then unplug it. There should be an app that counts down how much time you have left before the battery dies. I noticed I added one more 2 widgets and another "view" (desktop) and my battery is draining faster than ever.

Counting down in time is not going to be easy as it will change depending on what you have open at that time. The current battery programs are good as they track it over time/%age so give you a good idea.

It doesn't do exactly what you are asking but you can try installing battery-eye. It plots your battery level over time so you can ballpark how long your battery will last based on your own usage pattern.