When I made the switch to Mac OS X, I spent a lot of time trying to find equivalents to many of the applications I relied on in Windows. Here is a list of software that I consider essential, in no particular order. I'm sure there is stuff I'm forgetting, but this is a good start.

Office Applications
iWork 09: Apple's flagship productivity suite. Keynote is the best presentation software on the planet, period. Pages is a nice blend of word processing and page layout and I haven't had a need to use the Numbers spreadsheet yet.

Email, Calendar, Contacts
iLife 09: The standard applications that come with every Mac. The biggest adjustment is switching between Mail, iCal and Address Book instead of having everything in one place like Outlook on Windows.

Remote Desktop
Remote Desktop Connection: Microsoft's version of Remote Desktop for Mac OS X.

Font Management
Linotype Font Explorer X: Probably the best font manager I've ever seen on any platform and they give it away for free.

Video
VLC: the video player for everything that the Quicktime Player won't play.

MPEG Streamclip: Very useful video processing application. If you need to convert video from one format to another, this is where I would start.

Graphics Creation
Adobe Creative Suite 4: Still the leader on all platforms. Works the same as Windows with full file compatibility. Kind of spendy and starting to get bloated with features.

OmniGraffle Pro: Diagramming application that is much easier to use than Visio.

Pixelmator: Very streamlined application that can gives Photoshop-like capabilities at a fraction of the cost.

Web Development
Coda: Most awesome web creation tool if you like to mess with raw HTML or PHP.

RapidWeaver: Really nice tool for quickly putting together a site with pre-created templates. The fastest way to put together a nice looking static website with photo galleries, movies, blogs, etc.

Program Launcher
Spotlight: In Leopard, just hitting Command-Space and typing part of the name of an application is my favorite way of launching programs.

Quicksilver: Seemingly simple but with a wealth of depth, it quickly becomes an essential way of interacting with your computer. (With Leopard, I'm not using Quicksilver any longer)

Screen Shots and Screencasts
Skitch: Sweet little screenshot application with an interesting GUI and nice feature set.

Little Snapper: More extensive screenshot application that can snap entire web pages.

Text Editors
Text Wrangler: A free, very capable text editor.

Textmate: a very powerful text editor that developers and Unix geeks rave about. Very useful since it offers syntax coloring and code expansion for a large range of programming languages.

PDF Editing
PDFPen: useful application for editing, marking up, merging or splitting PDF documents. Much lighter weight and easier to use than Acrobat Pro.

Image Viewers
Preview: In Leopard, Preview has gained some great new features and it really takes the cake over any other image viewers I've seen.

Note Taking
Yojimbo: a really clean application that has become the place where I stash everything. I put notes, web clippings, reciepts, serial numbers, etc. and fills much of the functionality of Microsoft OneNote for me.

Backup
SuperDuper: Simple yet powerful backup application. Easily creates bootable images and can even backup via block-level differentials for fast incremental backups.

Drivers
SteerMouse: the standard Mac mouse tracking and acceleration algorithms are whacked, especially if you are coming from Windows. Steermouse gives you control over this as well as all of the buttons on your mouse without having to load crappy Logitech or Microsoft drivers.

Compression
The Unarchiver: expands virtually any type of compressed or archived file.

Database Management
Sequel Pro: good application for managing MySQL databases using a native Cocoa application.

FTP
Transmit: Top notch FTP program. Great UI and neat features.

Tweaks and Modifications
OpenTerminal: allows you to open a Terminal window with the path of the current Finder window. (Note: drag it to the Finder’s toolbar so you can use it in every window)
KeyFixer: Changes the behavior of the Home and End keys so they behave more predictably (if you’re used to the Windows behavior).

Windows Interoperability
MacDrive: this is actually Windows software for mounting HFS+ formatted volumes. Helpful if you are using Bootcamp.