How-To: Upgrade To Snow Leopard — The Right Way
Singular – a shareware program and automator action for finding duplicate files.
Omni Disk Sweeper – now freeware from our friends at OmniGroup, this will help us clean slim down our drives
Carbon Copy Cloner — a freeware/shareware app that makes a bit-for-bit copy of your hard dive. Usually used for making backups.
Extra hard drive or external hard drive (preferably Firewire) – Firewire is faster, and only Firewire drives are bootable, so get a Firewire drive if you want to make sure you can roll back to Leopard if anything goes wrong (or mission-critical software is incompatible with Snow Leopard). Otherwise, you can use a USB2 external drive