Something that protects the information and hard work. An external hard-drive (one of those 80-200GB babies) has been the best purchase I've made in the last year, so I think a gadget like this would be of great value to many.
I'd have to say my Pocket PC. I can hook it up to my cell phone and surf the web, chat on AIM, and check e-mail from anywhere. It's not exactly "for the office/home computer," but it's a really great gadget.
For office you should have a "All in one".. printer + fax + Scanner + copier, all in one small machine. Uses less space and costs very less relatively :thumbup:
Personally, I love the mobile Windows OS. There's so much you can do with it, and I can't wait for Mobile Windows v5 - I believe is what they were calling it.