I just installed Fedora Core 1 a few weeks ago and one of the many things that impressed my about it was the very good dictionary that came with it. I dont know if this part of Gnome desktop etc but it is very effective and seems to be pulling stuff from more than one source dictionary. It can also run in the taskbar as a little "lookup" box (see here).
It works estremely well and I find it very useful so I was just wondering does anyone know if something like this (free/open source) is available in windows?