A computer mouse is very useful to me for browsing the web. Personally, I think it's a lot more convenient than a trackpad.
A hand watch is really useful to me, because I'm the type of person who constantly needs to know what the time is, and you can literally wear it on your hand wherever you go.
I don't really have much to say for this one. It just makes sitting down at your desk for long periods of time a lot more comfortable and enjoyable.