This is a compilation of Mac software, paid or otherwise, that I personally use or am intending to try out. I’ve also listed web-based, online services.
Software I use
Firefox: I use this instead of Safari.
Google Gears: I have this Firefox add-on installed so that WordPress 2.7 administration and management screens load faster in my Firefox web browser. If you blog or edit posts on a regular basis, this speed increase is very welcome indeed. WordPress 2.7 gives you the option on whether to enable caching via Google Gears on a blog by blog basis.

Perian: Installing this piece of software allows QuickTime Player to be able to play formats such as AVI, DIVX and MKV, among others. It also adds subtitle support.
Document Palette: Allows you to create a blank document in the current folder that you’re in. You can define your own document templates. Read How To Create New Files In Apple’s Finder for more discussion on this topic.
Little Snitch: This software runs in the background and monitors all incoming and outgoing traffic with the purpose of helping you be aware of any packets generated by malware. In the screen capture below, OpenOffice wants to check with a service on their website, openoffice.org, which is a site I trust, so I would select the “Forever” option which tells Little Snitch to always allow traffic to updateext.services.openoffices.org on Port 80 TCP (http).
It costs $29.95 for a single license.

ScreenToaster: A free, web-based software that lets you capture and record your screen, which is great for screencasts, demos, tutorials or simply to show off your game high scores.
Isolator: Isolator is a great (and free) way of hiding your desktop and other screens. Read my brief review on Isolator.
Dropbox: Dropbox offers 2GB of free web-based disk space for your files. It installs a folder on your Mac, and that folder looks just like any other folder. I keep my work files in this Dropbox folder, and any changes to the files or folder structure automatically triggers a synchronization process that updates a set of exact copies on the Dropbox server, which are accessible on your other Mac, Windows or Linux machines. Read about how other people are using Dropbox creatively:
Links to related articles and blog posts
John Gruber talks about hardware and software he uses for work.