Open Source – Freeware – Shareware:

• Adium: the best instant messaging application for Mac. Integrates most of the protocols available out there: MSN, Yahoo Messenger, ICQ, Jabber, GTalk, Facebook and many more. There is even a Skype plugin. Largely customizable.
• Celtx: filmmaking and media pre-production tool, with collaborative support. Quite complete and versatile.
• Disk Inventory X: graphical disk usage viewer. Useful to see at a glance what the hell is going on with the hard disk free space.
• ffmpegX: graphical wrapper for mencoder. Really useful for converting videos.
• Growl: notification system that can be used by many other programs, like Adium, Skype, Safari, iTunes and so on. Many configuration options.
• Inquisitor: Safari plugin that overrides and greatly improves the Google search box.
• i-Installer: LaTeX installation manager. Already out of support, but still very useful.
• Plex (OSXBMC): great replacement for Front Row. It’s a port of the Xbox Media Center.
• Quicksilver: if I were to choose only one of the list, this would be it. Some people think that it’s only an application launcher, but it’s so much more than that. Optimizes the launching of applications and the execution of scripts (lots of them) using only the keyboard.
• Transmission: a really nice, simple and lightweight BitTorrent client.
• VirtualBox: x86 virtualization software. Lacks of 3D acceleration support, but it’s a great choice among others like Parallels.

Non-free:

• ComicBookLover: a catalog manager for electronic comic formats (CBR, CBZ), and an excellent viewer as well.
• OmniFocus: a GTD task manager. Well used, it’s a great tool to organize oneself.
• Papers: this is a great application for researchers. Organizes papers and articles by a wide criteria, and integrates a lot of search engines like Google Scholar or ACM, and is able to export a collection of papers in some formats, like BibTeX.
• TextMate: the ultimate text editor. For anything that is not writing Java code, this is what I use. Has lots of scripts for a huge amount of programming languages and text formats that really help in, for example, writing a large document in LaTeX or coding a Ruby on Rails application. Also incorporates a fair amount of keyboard shortcuts for those vi or Emacs addicts.
• viPlugin: it’s a plugin for Eclipse that allows for using almost all of the vi commands, therefore fixing one of the main drawbacks of this excellent IDE