Normally, I detest VPNs. They’re a good idea in theory – make you look as if you’re on your office network when you’re somewhere else – but my experiences of them at three different organisations has not been good. Fortunately I’m geeky enough to find alternative solutions using things like SSH, VNC, SFTP and WebDAV, which generally have performed much better. These days, though, if you have many mobile workers, it can be better to put your company services out there on the internet (with appropriate security measures, of course) where they can be accessed from your own network and from elsewhere.
However, more peer-to-peerish solutions like Hamachi could be very useful if, say, you want to back up your mother’s PC or print on your brother’s printer. As well as Windows support, there are console versions for Mac and Linux, and a new version with a GUI for Mac OS X.
Disclaimer – I haven’t tried any of these. Just an interested observer.