Some of the iLife applications can be downloaded for free.
iDVD and Garageband require buying a copy of iLife if you don't have the latest version.
Note that applies to the now current version of iLife.
If you bought a computer after it was released, the iLife applications should be up to date. (When you buy a Mac, the then current versions of the iLife applications are currently included. This probably won't change, but you never know.)
Depending on what Apple does with iLife, there could be another application in the next iLife revision or which ones cost money may change.