I've seen some complicated methods involving the Terminal for finding existing applications on your Mac that won't run with Lion, the latest OS from Apple. In case you don't know Apple is dumping support for "Rosetta", the technology that allows applications written for PowerPC-based computers to run on the newer Intel-based Macs. There are some notable casualties of this, including Quicken 2007 and Office 2004. You might want to know before you upgrade which of your applications aren't going to work anymore.
The GUI Way To Find PowerPC Apps
Apple being Apple you know there had to be a non-terminal way to do this right? Use the Finder’s Apple menu and select About this Mac. In the resulting dialog click the “More Info…” button. Wait a minute or two for the list to populate. Widen the window if necessary to show the “Kind” column. You can click on this column to sort by Kind. Then you can scroll to the “PowerPC” applications to see all your applications that won’t run under Lion.
I have about 400! applications that will no longer work. To be fair, most of these are in a folder that contains the contents of an old hard disk I copied over. Out of the entire list there were a only a few that I think might affect most people, couple of games, Quicken 2007 and Microsoft Office X. Your list will almost certainly be shorter and easier to review than mine. However, it's still worth doing to make sure you don't have any applications you can't live without.