Coolest must have apps and plug-ins for a Mac?

@YoBob what are your interests? What kind of work do you do? That would help narrow it down a bit.

Well, I’m a software engineer. However, this box will be used as a “family” machine for organizing pictures, video, the music library, basic home office type stuff, and general web surfing.

How about web development suites? I’ve been thinking about a couple of web based ventures I might throw into the either.

I also noticed last night that the included video application does not recognize .mpg and they want to charge for a plug-in that will. How about an open source transcoder or plug-in that will let me use all of the .mpg clips I have collected?

My essentials:
VLC – plays everything (I suggest using this to play your .mpg files rather than getting codec packs)
Unarchiver
Onyx – for cleanup
Flip4Mac – .wmv support
Cinch

For other stuff, I suggest checking out the App Store (should be installed already) and just looking through the categories. Lots of good (and fun) stuff.

Coda is excellent for web development.
Transmit is an FTP client with rave reviews.
Versions is a phenomenal subversion client.
Kaleidoscope is a really cool file and image diffing tool.
Code Collector Pro is a cool app that lets you save and organize snippets of code that you often use.
LittleSnapper is an excellent screenshot app and allows you to mark it up and easily upload an image to their servers. It also let’s you “snap” websites you like (i.e. you can archive the entire page, not just a screenshot) so you can keep a list for inspiration.

VLC works great for playing most video content out there (although the interface is a bit underwhelming).

Things is a highly regarded task-management app.
Delicious Library 2 is an app that lets you catalog all of your books, movies, video games, etc. It has a fantastic UI.
MemoryMiner is a neat app that lets you do really cool things with your family memories.
iWork from Apple is great and is able to act as a full replacement for MS Office for me.

MarsEdit 3 is an excellent blogging app.

For graphics:
Acorn is a reasonably-priced alternative to Photoshop, as is Pixelmator
Sketch and DrawIt are also great drawing apps both made by the same guy.

VoodooPad is a useful app that lets you sketch out your ideas in a wiki-like fashion. It’s not the prettiest app out there, but I’ve found it to be invaluable for helping organize my thoughts that I want to come back to later.

All of those apps should have free trials, so you can check them out. I have more development-oriented suggestions if you’d like, although it sounds like you’re more interested in family stuff for this machine. Be sure to check out the built-in software that is often overlooked. Automator is a cool app that lets you build what are essentially scripts, using visual tools. It might be a good introduction to programming concepts if you wanted to teach your kids (assuming you have them) the basics. The power of Preview.app is often under-appreciated. It is the most elegant .pdf viewer/editor I’ve come across on the desktop.

I can’t recommended Hyperdock enough. It is painful to use a mac without it.

http://i.imgur.com/4jh59.png

I really like Alfred, which a sort of launcher thing meant to replace Spotlight or just make launching apps a lot, lot easier.

You can pretty much use it just so you don’t need tons of apps in the dock, or with the powerpack version you can actually browse file structure like in the finder. It’s pretty powerful.