I thought it would be interesting to compile a list of apps that are Intel-only.
Make a note of the version of the app if known and add [partially] if only some specific features are not PowerPC compatible.
I'll leave games out of this for now - that's a different story.
Let's get the ball rolling:
Adobe Soundbooth
Adobe Premiere
Adobe Encore
Boxee
Garageband '09 [Partially: Learn to play feature intel-only]
OS X 10.6 – Snow Leopard
Parallels Desktop
VMWare Fusion
i couldn't find exact specs, but i'm certain that Red Alert 3 is only for Intel macs.
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My List of Toys: Samsung 50" DLP, 360 Elite, PS3 500GB, MacBook Pro (2.5ghz,6gb,500gb), 3G Iphone, 60gb Ipod, Samsung NC10 (1.6atom, 2GBramm, 120GB).
Mac Specs: MacBook Pro 2.4GHz Intel Core 2 Duo 4GB DDR3 RAM
VirtualBox is one of my personal favorites. Much like VMWare and Parallels it runs basically any OS (except for Mac). The difference is that it is free and extremely simple to use. Right now I am using it for Linux and Windows so I can use Linux for C++ and Windows for whenever I'm bored and want to be reminded why I switched to Mac.