Hamachi allows you to establish secure direct connections to computers that are not accessible otherwise due to the restrictions in network setup.
I just stumbled across another P2P app that facilitates personal filesharing across the Internet without lots of firewall traversal hassles called Hamachi. I haven’t tried it yet, but I’m interested in learning more.
I tried Grouper and dropped it because of it panders to the RIAA by streaming audio files at a low quality, rather than just treating them like any other file.
Then I tried Foldershare to make some files available between my home and work machines, and also to share various files with my brother in Utah. I like FolderShare’s simplicity, and that it works well for individual scenarios (like the way I used it to make a few files available at both home and work), and for group sharing and collaboration (like the way I’m sharing some files with my brother).
Groove supports similar functionality in their products. It will be interesting to see if Skype uses the P2P infastructure of Skype to enable other communications applications like this in addition to their IM and Voice features.