Wireless backups with Time Machine without Time Capsule

Solution 1:

Here's a great tutorial on how to do just that:

http://hupio.wordpress.com/2008/04/27/osx-timemachine-and-sambawindows-share/

Not entirely different from sharing out a folder on your Windows machine and connecting via SMB. :P

Sharing external storage between different operating systems?

Solution 2:

And while such solutions will probably use a sparse bundle, a detail to remember: if you ever make backups of really large files, then deleting those files from the backup (either manually, or when Time Machine removes expired daily and hourly backups) will not automatically reclaim the disk space -- until Time Machine really needs it. At that time, Time Machine might first remove a few more backups before realising what's going on.

See details at What is Time Machine doing? on Server Fault.