Windows: We've mentioned one way to sync your iPhone with multiple computers, but if you'd rather not download a hex editor just to manually hack your iTunes libraries, free utility PhoneAble will do it all for you with just a few clicks.
So many of us have more than one computer these days, but if you want to keep your iPhone or iPod Touch updated with information from more than one computer, Apple doesn't really provide a solution. Luckily, even if you're not comfortable hacking, you can easily grab data from two computers. Just sync your iPhone with your main machine, install the app on your other computers and run through the steps it provides. It will set your library key on each computer to match the one on your iPhone, thus allowing you to sync whatever data you want.
PhoneAble is a free download for Windows only. I only have one Windows computer at my disposal right now, so test it out and let us know if it works for you in the comments.