system
10
Really?
I do.
Clock
CPU Meter - not the most accurate I think, but helpful to me none the less…
Calendar
Weather
And of course, avast! 
I suppose it depends on how much screen real estate you have…on my vista system I don’t have to use fulscreen, so I use the sidebar, but my laptop is obviously smaller, so I only use Clock, Cpu meter and the smaller version of the avast one (win7 only) But I only see them when everything is minimised.