Below you’ll find an comprehensive list of very useful MacOS X applications.

They’re split up in several sections, ordered alphabetically.

Media Applications

QuickTime Codecs and applications for watching, editing or manipulating media like movies, audio or pictures.

Application Description
DivX DivX codec for QuickTime and DivX webplayer
EyeTV EyeTV is software to watch TV on your mac using a hardware tuner.
ffmpegX ffmpegX is a DVD, SVCD, VCD, CVD, VOB, DivX, XviD, H.264, PSP, iPod, MP4, MOV encoder for Mac OSX
Flip4Mac QuickTime codec for playing windows media (wmv, wma) on your mac
HandBrake HandBrake is an open-source, GPL-licensed, multiplatform, multithreaded video transcoder. In other words: a DVD ripper.
VLC VLC media player is a highly portable multimedia player for various audio and video formats (MPEG-1, MPEG-2, MPEG-4, DivX, mp3, ogg, ...) as well as DVDs, VCDs, and various streaming protocols. It can also be used as a server to stream in unicast or multicast in IPv4 or IPv6 on a high-bandwidth network.
Perian Perian is a free, open source QuickTime component that adds native support for many popular video formats.
RealPlayer Video player for Real encoded videos
Snapz Pro X Snapz Pro X allows you to effortlessly record anything on your screen, saving it as a QuickTime movie or screenshot.

Networking Applications

Applications for getting around on the network, it includes programs for chatting, surfing, email, administrating and more.

Application Description
Adium Adium is a free instant messaging application for Mac OS X that can connect to AIM, MSN, Jabber, Yahoo, Google Talk and more
Chicken of the VNC Chicken of the VNC is a VNC client for Mac OS X.
Colloquy Colloquy is an advanced IRC, SILC & ICB client
Cyberduck Cyberduck is an open source FTP, SFTP, WebDAV and Amazon S3 browser
FireFox The Firefox Web Browser is the faster, more secure, and fully customizable way to surf the web.
NetNewsWire Free RSS and Atom reader for your Mac
Remote Desktop Connection With Remote Desktop Connection Client 2, you can quickly, simply and securely connect to Windows-based PCs to access Windows-based files, applications, devices, and networks from your Mac.
Skype Make internet calls (VOIP) using Skype
Thunderbird Email application
Transmission Transmission is a fast, easy, and free multi-platform BitTorrent client
Unison Unison is an Usenet (newsgroups) reader which allows you to download music, pictures and videos
XMeeting XMeeting is the first H.323 compatible video conferencing client for Mac OS X. In addition, XMeeting also supports the popular SIP protocol used by many VoIP applications.

Productivity Applications

Office applications, image editing

Application Description
GIMP.app The GNU Image Manipulation Program (gimp) for OS X.
NeoOffice NeoOffice is a full-featured set of office applications (including word processing, spreadsheet, presentation, drawing, and database programs) for Mac OS X. It is compatible with Microsoft Word, Microsoft Excel, Microsoft Powerpoint. It is based on the OpenOffice.org office suite
OpenOffice OpenOffice.org is the leading open-source office software suite for word processing, spreadsheets, presentations, graphics, databases and more. It is compatible with Microsoft Word, Microsoft Excel, Microsoft Powerpoint
Pixelmator Pixelmator, the beautifully designed, easy-to-use, fast and powerful image editor for Mac OS X has everything you need to create, edit and enhance your images..

Tools

Just a list of useful tools.

Application Description
ByteController Quickly control iTunes from your menu bar
Caffeine Caffeine is a tiny program that puts an icon in the right side of your menu bar. Click it to prevent your Mac from automatically going to sleep, dimming the screen or starting screen savers.
GrandPerspective GrandPerspective is a small utility application for Mac OS X that graphically shows the disk usage within a file system. It can help you to manage your disk, as you can easily spot which files and folders take up the most space. It uses a so called tree map for visualisation.
Growl Growl lets Mac OS X applications unintrusively tell you when things happen.
Mactracker Mactracker provides detailed information on every Apple Macintosh computer ever made, including items such as processor speed, memory, optical drives, graphic cards, supported Mac OS versions, and expansion options. Also included is information on Apple mice, keyboards, displays, printers, scanners, digital cameras, iPod, Apple TV, iPhone, Wi-Fi Cards/Base Stations, Newton, and Mac OS versions.
MagiCal MagiCal is a menu-based clock and calendar. It features a huge range of configuration options for how the time and date are displayed, and can operate either in conjunction with, or as a replacement for the built in system menu clock.
OnyX OnyX is a multifunction utility (maintenance, optimization, and personalization).
It allows you to verify the Startup Disk and the structure of its System files, to run misc tasks of system maintenance, to configure some hidden parameters of the Finder, Dock, Dashboard, Exposé, Safari, Login window and of some of Apple’s own applications, to delete caches, to remove a certain number of files and folders that may become cumbersome and more.
UnRarX UnRarX is a Mac OS X Cocoa application that allows you to expand rar archives and restore corrupted or missing archives using par2.

Development Applications

Several tools which help me with my development.

Application Description
Developer Color Picker Developer Color Picker is A custom color picker designed specifically for developers. It integrates with the standard color picker. Currently supports NSColor, UIColor, CGColorRef, CSS and HTML styles.
Free Ruler Free Ruler is a free screen ruler for Mac OS X.
MySQL GUI Tools The MySQL GUI Tools Bundle includes the following products: MySQL Administrator, MySQL Query Browser and MySQL Migration Toolkit 1.1
OmniGraffle OmniGraffle Professional includes a powerful toolset for advanced document creation and editing options. For the pro user who’s looking for a full-featured diagramming and design tool.
pgAdmin III pgAdmin III is a GUI for PostgreSQL administration and development.
TextMate TextMate brings Apple’s approach to operating systems into the world of text editors. By bridging UNIX underpinnings and GUI, TextMate cherry-picks the best of both worlds to the benefit of expert scripters and novice users alike.

Power Users

Mac OS X Tools for Power Users

Application Description
IPSecuritas IPSecuritas is a free IPSec client for Mac OS X. It supports virtually every available IPSec compliant firewall (including FRITZ!Box by AVM), allowing you to connect safely to your office or home network from any location on earth.
Lingon Lingon is a graphical user interface for creating an editing launchd configuration files for Mac OS X Leopard 10.5.
MacFUSE MacFUSE allows you to extend Mac OS X’s native file handling capabilities via 3rd-party file systems.
Mac GNU Privacy Guard Several GPG (GnuPG) Utilities for your Mac
MacPorts The MacPorts Project is an open-source community initiative to design an easy-to-use system for compiling, installing, and upgrading either command-line, X11 or Aqua based open-source software on the Mac OS X operating system
Porticus Porticus is a Cocoa GUI for the MacPorts package manager.
Quicksilver QuickSilver is a unified, extensible interface for working with applications, contacts, music, and other data.
Secrets Secrets is a database of hidden settings for Mac OS X
SSH Tunnel Manager SSH Tunnel Manager is a GUI for the ssh command when used to open tunnels between two hosts.