Most of the time 3-6 windows open (but occasionally over 9).
Most of these are usually browser windows, but I tend to close them as "rogue" javascript or flash *kills* the battery life.
GPodder remains open almost permanently as listening to podcasts is one of the primary uses my N900 gets

On the desktop, Diary and Foreca Weather Widgets, and on another homescreen the RSS widget.
Online for Skype, GTalk & Ovi chat most of the day (although this means I now travel everywhere with a charger since losing my extra battery),
Some sort of battery "hot-swap" solution would be most welcome (i.e. allow the main battry to be swapped out without having to power-down)

The *MOST* useful thing is the CPU meter that runs in the Battery & reception indicator area.
This shows me when some web page is causing 100% CPU so to be patient. It nearly always is a web page with lots of javascript.