I haven't seen anyone mention Mac software, so I'm gonna:
Audio
iTunes (v4.2) - Mac and Windows. AAC support on both platforms, powered by
QuickTime, great organizer, burner, player. Free.
SparkME (v2.81) - Mac ONLY. Freeware version of their Spark editor. Has been discontinued, so hurry and get it before they realize you can still get to it at the link I've attached.
SoundStudio (v2.11) - Mac ONLY. Waveform editor, ships with some consumer Mac models. Shareware.
Video
DivX (v5.1.1) - Mac and Windows. Think we all know what this does. Free.
ffmpegX (v0.0.9a) - Mac ONLY (but powered by multi-plaform binaries), encodes video into various formats, including SVCD, DivX, XviD, etc. Donation-ware.
0SEx (v0.0110a01) - Mac ONLY, extracts stuff off DVDs, ready for processing by ffmpegX. Free.
VLC Media Player (v0.6.2) - Lots of platforms, plays DVDs, CDs, VCDs, DivX, WMA, MP3, Ogg Vorbis... only complaint is that it's still a touch buggy and needs an update soon. Free/open-source.
BTV Pro (v5.4.1) - Mac ONLY. Works great with my iSight cam, has lots of options for capturing video or still images. Shareware.
"Productivity"
Microsoft Office X (v10.1.5) - Mac ONLY (but compatible with the Windows version). Yeah, I know what people say about Microsoft and about Office, but I get it for free since we're site-licensed for it, so I can't be bothered looking into anything else. Nor is it really worth my while... Commercial.
SubEthaEdit (v1.1.5) - Mac ONLY. I love this. You haven't edited text until you've done it collaboratively over the network. Supports Rendezvous (aka ZeroConf), hydra:// (its previous name) URLs, and has a live web preview via WebKit (aka the Safari KHTML-based frameworks). Free.
BBEdit Lite (v6.1.2) - Mac ONLY. Recently displaced as my editor of choice by SubEthaEdit, but still very good to have around. Discontinued, but remains on some mirrors/archives - do a search and you should find it. Might even be on the barebones.com FTP server. Free.
vi/sed - I grew up on vi and never really got introduced to anything else (except pico), so have stuck with this. More recently, discovered sed as a useful way of enabling shell scripts to modify files. Free.
Dreamweaver (vMX 2004) - Mac and Windows. Like Microsoft, I get it for free due to our license. I can write raw HTML in a plain text editor, but sometimes I really can't be bothered, particularly when I want to use some basic Javascript behaviours, but I do keep the Split view open. Commercial.
Graphics
GraphicConverter (v4.9.2) - Mac ONLY. Versatile image converter, viewer, browser, editor, slide-show viewer. Converter has a useful batch mode so you can set a bunch of images being resized then cropped then saved to a format... Does stuff Photoshop doesn't. Shareware.
Photoshop (vCS) - Mac and Windows. I don't live in Photoshop like I know some people do, but there are plenty of things I use it for. Commercial.
Internet
Mac OS X Mail (v1.3.2) - Mac ONLY. I've played with lots of different email programs (Mozilla, Eudora, Thunderbird, Entourage...) but I keep coming back to this one, since it's fast, has good spam detection and doesn't use up the whole screen. Built into Mac OS X 10.x.
Mozilla (v1.5.1) - Multi-platform. I have Safari installed of course, but it doesn't do a bunch of things that Mozilla does (Mozilla's image, cookie and pop-up blocking is first-rate). A bit "heavy", but still reliable.
Mozilla Firebird (v0.7.1) looks interesting but still seems immature. Free/open-source.
Camino (v0.7) - Mac ONLY. Mac OS X up front, Mozilla in the back. Gecko embedded in a Cocoa application. Great browser, would be nice if it got some of Mozilla's blocking features. Has gained features in the nightlies, but an official release hasn't happened since March 2003. Needs to be officially updated. Free/open-source.
iChat AV (v2.0) - Mac ONLY. Prefer to the official
AIM client. I'm disappointed in some of those multi-protocol apps like
Proteus and
Fire (the Mac equivalents to Trillian, etal) due to their bad support (ie: non-existant) for firewalls and proxies. iChat looks good, works good, is integrated well, and it was free with Mac OS X 10.3. Built-in to Mac OS X 10.3/Commercial (sold separately for 10.2.x users).
Transmit (v2.6.2) - Mac ONLY. Great Mac OS X FTP client. Better than Fetch. Looks and works like a Mac OS X application should. Shareware.
Conversation (v1.4) - Mac ONLY. I've been using IRC for 10 years, and me and BitchX get along just fine. But I think this is just such a cool idea - puts an iChat-style face on IRC. Free.
Utilities
Terminal - Mac ONLY. Keep it open all the time. It's my command-line, my SSH client, my some-time FTP client... wouldn't be without it. Built into Mac OS X.
Toast Titanium (v6.0.2) - Mac ONLY. Burns lots of different types of CDs and DVDs, including PC-compatible discs, and the clever custom-hybrid type (separate Mac and PC partitions on the one disc). iTunes and Finder are good for knocking up quick discs, but Toast is the champ. Commercial.
StuffIt Deluxe (v8.0.2) - Mac and Windows. Handles every archive format you can throw at it. Discovered it lets you browse TAR files, just like you can with SIT, SITX and ZIP files. Commercial.
Carbon Copy Cloner - Mac ONLY. THE tool for making Mac OS X image files. The same guy also makes
NetRestore, for restoring image files through NetBoot. Indispensible for imaging and integration. Has a forum and some work instructions for a variety of integration-related issues. His software is free for education users. He's so smart he now works for Apple.

Free (for education).
Virex 7.2.1 - Mac ONLY. Comes as part of a .Mac membership, but we get it through our corporate NAI license. In 14 years of Mac computing, I've never once had a virus on my Mac, but I've no complaints about Virex (except the fact that only administrators can update the DAT - yes, the DAT itself should be protected, but the update mechanism should not). Commercial.
Mactracker (v2.0.6) - Mac and Windows. Specs on every Mac, Mac clone (yes, they did exist) and Apple-branded Mac-compatible peripheral from the original 128k Macintosh up to the iBook G4. Helps me maintain the illusion that I know everything about Macs. Free.
Apple Remote Desktop - Mac ONLY. Couldn't support and manage all the Macs that I do without it. Commercial.
Remote Desktop Connection - Mac ONLY. The Mac equivalent of Terminal Services Client. Good for managing Windows servers from my Mac, or running some Windows-only software from my Mac workstation. Free.
Other
Mac OS X 10.3 (Panther) - Mac ONLY. Love this upgrade. Expose, Preview (who needs Acrobat Reader?), iChat AV, Finder, improved proxy support, better layout of many apps (eg: Network Prefpane, the Internet Connect application), upgrades to practically everything (including the UNIX/BSD layer), speed improvements all around - wouldn't go back to 10.2 for anything. And the new ActiveDirectory plug-in makes network auth/home directories a lot easier than trying to learn LDAP from scratch. Plus it has all the usual UNIX stuff (it _is_ the world's highest volume shipping UNIX) like built-in Apache, Samba, PHP, Java 2 (1.4.1)... out of the box, and Apple's free Xcode IDE for developing apps in Java, C++, Objective-C, etc (think MS Visual Studio, but free). Commercial.