File syncing across multiple operating systems [closed]

What would be the best way for me to sync multiple folders in various locations across Windows 7, XP, Ubuntu, OSX computers as well as firefox addons (that might just be a folder anyway, i'm not sure)? Online backup would also be appreciated, as would a friendly and pretty interface (I love Dropbox it's just not up to everything I want).

Thanks!

Unison Sonds like the tool you are looking for. It allows for both, interactive GUI-based, and scripted solutions.

I use rsync.

It is installed on most Linux distributions by default, and Mac OS X as well. There's a Windows version that comes with Cygwin, or available separately from the full Cygwin stack.

For an online solution, I would look into rsync.net.

For Firefox and addons alone, try the FEBE extension.