SugarSync is my favorite way to keep all my important files and folders synchronized. Just install it on each of your PCs or Macs, and tell it which files and folders to sync--it then works behind the scenes. As you edit a file, SugarSync uploads the file to cloud-based storage, and then syncs it to your other devices. You can access the files from any computer with Internet access, and from an Android or iOS device too.
SugarSync is free for two computers and for mobile devices, for up to 5GB in storage. If you want more than that, you'll have to pay. The service has several price tiers, depending on the amount of storage you want, from $50 a year for 30GB to $400 a year for 500GB.
Windows Live Mesh
Microsoft's Windows Live Mesh is another great tool for keeping files and folders coordinated. It's free, and it works in concert with Microsoft's cloud-based storage service SkyDrive to keep your PCs and Macs synchronized. You get up to 5GB of cloud-based storage. Just like SugarSync, it sends files to the cloud storage and then syncs the files from there, so you can access the files not just on your own PCs and Macs, but wherever you have an Internet connection. Not only that, but you can also access your PC or Mac from Windows Phone 7, Android, and iOS devices.
If you browse the Web on multiple computers, or if you use multiple browsers, trying to keep their bookmarks in line is a huge hassle. With Xmarks, however, that's no longer a problem. This tool syncs bookmarks between multiple PCs and Macs, and works with Firefox, Internet Explorer, Mac OS X Safari, and Chrome.
The software and service are free for computers, but if you also want to sync your bookmarks with IOS and Android devices, you'll have to pay $12 per year.