Back in Lion, System Preferences > Software Update had some cool information under the Installed Software tab. You could check and see everything you'd installed using OS X's built-in updating system and sort by the names or the installation dates to find what you were looking for.
This was very good to know when problems cropped up, as you could see if your trouble began after you'd installed a particular update and do your troubleshooting armed with that knowledge. However, if you open System Preferences > Software Update under Mountain Lion, this is now what you get:
Aw, sad. Where'd my list of installed software go? Luckily, it's still around—it's just kind of hidden. To get to it, click on the Apple Menu, hold down the Option key to make About This Mac switch to System Information, and select that to open the program. Then scroll down the list on the left of the window and look for Installations, which is under the Software section.
When you select that, you'll be rewarded with the list of your installations, but with a handy new twist—both Mac App Store and third-party installations are now included, and each one lists where it came from.
Neat-o! So if you're having trouble, you can use this list to figure out if a potential culprit was installed when your problems began by clicking on the Install Date header to sort by that column.
I gotta admit that I don't understand why this got moved, but hey—it makes System Information even more useful. And it was already pretty darned useful. For more information, check out Sandro Cuccia's excellent article on what System Information is and why you should care.