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For my main Mac Pro, I use for just writing and recording music with Pro Tools. That's pretty much it. I also have a 2nd Mac Pro that I use for Windows 7. There's still some software that's only for Windows that I like, plus I like using Steam to play the occasional PC game on it. Apple makes the most reliable Windows machines in my opinion!
 
Final Cut Pro X, Compressor 4, BIONIC Manager (Einstein@Home Distributed Computing), Google Earth, and the normal apps like Safari and iTunes, etc.
 
It's pretty much my main system, so I do everything on it:

Web surfing
iTunes
Gaming
Aperture 3
Development (lots of VMs running concurrently)
 
Lets See...:

XCode
Maya
3ds Max
Final Cut Pro
Shake
Logic Pro
Soundtrack Pro
Motion
After Effects
Photoshop
DaVinci Resolve
Other general Software and Graphics Development tasks
Oh, and ofc iTunes, Safari, Mail and iChat.
 
I bought mine mainly to just consolidate from multiple physical machines into a few VM's:

Win 7 x64
Win 2008 r2
CentoS 6.0
Ubuntu 11.04

It's awfully nice only having one machine instead of the 4 I was using.
 
Photoshop (Web photo etc)
Illustrator
InDesign (layouts and comic books)
Comic Life 2 (One of my favorite little programs for creating great little comic books of our little adventures and then shoot them out to friends and family and to our iPads for reading!)
Web development with Netbeans and running vmware with windows xp for MS Web development
iTunes
Ripping music
Ripping DVD's
Learning Linux (Ubunto via vm ware)
PDF Creation
Heck just about everything I do....
 
My 8 core is used for hardcore 3D rendering. The program relies on the CPU and not on the GPU. All 16 threads are used 100% during renderings. Without this machine my renderings would take days to finish.
My quad core is used for illustrator, photoshop design work. And also bases as a server for security cameras, and file server for the rest of the computers in my studio.
Best thing about the mac pros: dem expansion capabilities :D its great to run several terabytes and have no cables lying around :)
 
I use my first gen MP for everything, surfing, email, Lightroom, managing my digital music and video collection, Parallels for when I need windows for something the Mac can't do, games, and everything else I failed to mention.
 
Applications
Pro Tools 9.0.5b
Propellerheads Reason 5 and Record 1.5
Final Cut
Photoshop
Illustrator
Dreamweaver
Roxio's Tivo Connect
RipIt
Handbrake

Propellerhead's is releasing Reason 6 in a month, and it has gone 64-bit. Should be interesting to see how it performs.

Pro Tools 9.0.5b is working on Lion for me right now. I have not loaded third-party plug-ins yet, though.
 
If you are wondering why I have a MP it's mostly b/c I need my main computer to be as reliable, user repairable, and upgradable as possible in addition to raw speed.

I could do fine w/ a high end iMac but I'll never buy one as long as it's a PITA to access the hard drive. I also don't like the fact that if the monitor goes I have to take the entire machine in for repair, and also repair might not be cost effective.

Ha, I thought I was the only one who bought it for exactly these reasons...

It may cost a lot, but it has peace of mind written all over it. I've had my MP now for over 3.5 years, and it's STILL the "PC" I always wished I had.

Silent
Able to multitask like a fiend (whether encoding 10 movies or converting 100 songs at once, it just doesn't slow down and more importantly doesn't slow ME down)
Streaming music and videos to my iPad from wherever I am
Photoshop
Web surfing
All the day to day stuff...

And usually all of this going on at the same time - I keep at least 10-15 programs running on the MP at any given time, 24/7.

8 core BEAST :D
 
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