Below is a list of freely available software that I would recommend for all Mac users.

Must Haves

• Alfred - If you install nothing else, install this. Alfred makes it easy for typers to find applications, folders, etc.

Utilities

• f.lux - Save your eyes at night
• OmniDiskSweeper - A simple utility to find where you're using all your disk space.
• SuperDuper! - A great disk cloning utility. The free version is quite useable for the occasional backup. You'll want to purchase the full version if you're going to use it heavily.
• TechTool Delux
• Tunnelblick - Great open source VPN client for OSX. Unobtrusive and very functional.
• coconutBattery - Found out the health of your battery
• The Unarchiver - Great add-on to support additional compression formats (rar, etc.)
• iOS Backup Extractor - Simple utility to get all your files (like voicemails) out of your iTunes backup

Graphics

• Gimp - A great free image editor
• Seashore - Seashore takes core parts of the Gimp and brings it into OSX's native look and feel. Note that while quite useable, it's not as complete as the Gimp.
• Skitch - A Simple photo annotation utility. Create arrows, fun doodles, etc.

Productivity

• Evernote - A great way to sync notes, images, etc. between all your devices
• OmniDazzle - A simple utility to highlight certain areas of the screen. Great for presentations or just entertainment.

Finance

• GnuCash - Relatively simple open source finance tracking application. The only personal finance tool I've seen (free or commercial) that really makes use of double-entry accounting.

Media

• VLC - While Quicktime Player is quite nice, sometimes you it doesn't understand some files. VLC is usually the answer.

Medical

• OsiriX - If you've got digital XRays or MRIs, this free utility will let you view it

Miscellaneous

• Google Earth - Take of a virtual tour of the big blue dirt ball
• uTorrent - A tiny, no-nonsense bit torrent client