Everything Apple (the Mac heads thread)

Once time machine has finished backing up, it'll have a folder the size of the used part of your hard drive, but you'll be able to use the remaining space as you see fit.

This isn't quite accurate, if you use the remaining space as you see fit you risk the following during a Time Machine backup:

Over-filling the drive once the TM process has already begun and corrupting that (one) backup once it runs out of space that was initially labelled as free space.
Corrupting the backup drive entirely by changing the permissions/name/hierarchy of that one folder by mistake.
Reading/writing to the drive excessively whist TM is doing it's thing... this usually corrupts that (one) backup but can also require the drive to be re-formatted for TM if you're very unlucky (if you manage to stall the TM process as it's clearing up/assigning the new file allocations for the drive).

Perhaps not such a big deal for casual users (as they'd simply start to backup from scratch again), but I would be pissed if I lost my old backups (indexed amends by date).