iPhone: Skype for iPhone successfully carries the principles of quick and easy communication across from the desktop to the mobile, allowing users to make free or cheap calls anywhere in the World.You can use it to make free calls over Skype only over WiFi, and SkypeOut calls on 3G to cellphones and landlines using Skype. I think it will be a long time before carriers allow Skype-to-Skype calls over 3G, though.
Mac: Many people use Parallels for hosting Windows inside OS X but VirtualBox (now updated to version 3.0.6) is a free open source alternative that does a pretty decent job too. With VirtualBox, you can install any version of Windows (as long as you have the boot disk), Linux, Solaris and just about any other operating system you can think of.
Windows: Tired of your old music collection? With Spotify (new version 0.3.19) you can have access to thousands of songs, bands and music styles anywhere, anytime. This awesome app streams high quality music to your computer for free, includes bio data about your favorite bands and lets you share playlists online with your friends.