While these aren't the only alternatives to specific applications, it should get you off to a good start. You might also like to try searching a software download site like MacUpdate or VersionTracker.

Instant Messaging

Adium X
Free and highly customisable, Adium X connects to many instant messaging services including: AOL IM (.Mac and ICQ), Jabber (GoogleTalk), MSN, Yahoo, Bonjour, Gadu-Gadu, LiveJournal, Novell Groupwise, QQ and Lotus Sametime. Overall, an excellent all in one solution.


IRC Client

X-Chat Aqua
Free open-source IRC client.

Colloquy
Another free IRC client for OS X.


DVD Ripping

HandBrake
Even after a 14 month hiatus, HandBrake remained one of the top choices for many Mac users when ripping DVDs. The project has recently started up again, now providing features like surround sound decoding, chapter markers and settings to rip your DVDs into AppleTV and iPod compatible videos.

MacTheRipper
This application is also quite popular. While it doesn't directly convert DVD movies into other formats, it copies content off a DVD after removing the copy protection to your hard disk. You can then burn the content to another DVD. Great for making back-ups of your movies.


Burning CDs and DVDs

Burn OS X
Open source application that can burn CDs and DVDs in many formats.

Toast
Not free, but a very popular and complete application to tend to all your disc burning needs.

Disco
Low cost application that can perform basic burning tasks.


Backup Tools

SuperDuper!
A lot of your data is probably invaluable - so even this application isn't free, it's a small price to pay for peace of mind. Don't be mad at yourself later - always make backups!


Torrent Client

Transmission
From the people that brought you HandBrake, comes Transmission. Lightweight and easy to use, Transmission is definitely a great application to serve your torrent requirements.


Windows Media Player

VLC
VLC is an audio/video playing application that supports many formats including Windows Media Video.

Flip4Mac WMV Plug-in
The Flip4Mac WMV plug-in will actually allow you to play Windows Media Video files inside of QuickTime.


FTP Client

Transmit
Transmit won an Apple Design award in 2005 for "Best use of Mac OS X Tiger Technologies. It's not free, but certainly an excellent application worth the small cost.

Interarchy
A very comprehensive collection of tools for uploading, downloading, editing and more.

CyberDuck
It's free and easily performs FTP functions.


Office

Office:mac
Not free, but from Microsoft themselves - and more likely to be immediately compatible with your various Office files.

OpenOffice
A project to make an open source suite of applications that are compatible with and offer similar functionality to Microsoft Office.

NeoOffice
A variant of OpenOffice.

iWork
Apple's offering. Currently only contains a word processor and slide show application.


Editing Text

TextWrangler
Free application that, well... edits text. Useful for when you have a more advanced requirement than the OS X included "TextEdit" can handle.

TextMate
Not a cheap application, but a very advanced text editing tool.


Web Browser

FireFox
This application is slowly eating away at Internet Explorer's massive market share, which is sign that it is an excellent program. A nice, free alternative to the OS X included "Safari".

Camino
Another free alternative browser.


Compressing/Uncompressing Files

StuffIt Expander
Free and supports many file compression formats. This application only expands files.

StuffIt Standard/Deluxe
These applications can compress files in addition to uncompressing them.

The Unarchiver
Free and supports many compression formats. This application only expands files.


Run Other Operating Systems

Parallels
Amazing virtualisation software for OS X that will not only run pretty much any version of Windows - it also runs all kinds of Linux distributions as well. With this software, you can run another operating system at the same time as you are in OS X.

VMware Fusion
Amazing virtualisation software for OS X that will not only run pretty much any version of Windows - it also runs all kinds of Linux distributions as well. With this software, you can run another operating system at the same time as you are in OS X.

BootCamp
Apple's solution. Currently supports Windows XP and Vista. This is a "dual-boot" solution, so you can only run one operating system at a time.


Converting Video

Visual Hub
Visualhub converts anything to everything. It has inbuilt settings for optimising video for TV, iPod, iPhone, DVD and many more. It costs about $25 AUD.

iSquint
Visualhub's younger sibling, iSquint will do the same jobs as visualhub but with fewer export options.

Nice. i'll sticky this, and get rid of the msn/adium sticky from earlier tonight. Thanks heaps [MREVO8]

a very helpful post to a n00b such as myself
thanks and gw

Add iWork to your Office list

New category: text editors -
TextEdit mangles code something fierce! It's really more of a gutted word processor than a "text-editor" and not much use if you're working with scripting or web development.
Give TextWrangler a bash, free, clean, and feature-rich!

SuperDuper under a backup tools category.

handy. i was just coming to ask what ftp client to use... and its there allread... almost like you could read my mind.

You could add a cd burning program category as well..

Toast and Burn OS X

Updated. If you have any more suggestions for existing categories or a new category, post them here and I'll add them.

Camino is great...

I've never understood this. I might say "I installed Camino..." (which I did). "It's so good it crashes at ramming speed..."

That's impressive - I run Camino 24/7, usually with 10+ tabs open. The main reason I use it is because it's the only browser that doesn't leak memory and won't crash after several days of heavy browsing. Safari clogs up and gets re a ll yy sl o w, and it is about as stable as a drunk on a unicycle when you have lots of tabs/windows open. Firefox eats memory like it's some sort of drug. That leaves Camino.

It uses the same rendering engine as Firefox anyway, so unless you did something very strange, the crash was most likely a very badly coded webpage that would also crash Firefox (on Windows and Mac).

No arguing... let's keep this on topic please.

Its not free but Textmate is a kick arse text editor, and was one of the things that pushed me over the line from Linux to Mac. Its great for programming, html, css, and general purpose text editing

Sorry, MREV08.

I would add Interarchy to the FTP clients list - many people (myself included) prefer Interarchy. Also, I would add Textmate to the text editors - Textmate isn't cheap, but it's kind of like a samurai sword of a text editor, that combines the best elements of Mac and Unix philosophies. It's very simple, but incredibly extensible. It has very nice shell integration and all sorts of other things that programmers love.

There are some other little utilities and things that I love, but I'm not sure if you should put them in the sticky or not.... MenuMeters gives you system stats in your menu bar, and USB overdrive can make some 3rd party USB mice and keyboards suck a little less. There are heaps of other things, but they're probably not "essential", they're more "luxury" programs!