Microsoft Office 98 started the trend of putting a folder on the install CD with the label "drag this folder to your applications folder to install," and since then, many other developers have adopted this method for installing software.
This hint is useful for folks that don't use any kind of intricate filing system for their applications, and are content with letting them all live in the "Applications" folder at the root of your boot disk.
Once you've mounted the disk (and opened its window, if it doesn't auto-open) that contains "drag me.app" (or "drag me folder"), hit Command-B. This will reveal the toolbar in that particular window, and, presuming you haven't customized your toolbar prefs too much, you should see an "Applications" icon. Simply drag the app onto that icon, and boom, it'll be copied into your Applications folder, ready to rock. One step installs without having to open any new windows.
[robg adds: Yes, experts, this is a relatively straightforward hint, but many people don't think to enable the toolbar on disk image windows. Also, if you've enabled pop-up folders, there's no reason this applies to only "simple" filing systems. Just drag and hover (or drag and hit the space bar), and then navigate to whatever spot you want to use. I use a variant of this trick for all my drag installs, except I have a "My Computer" alias at the left edge of my toolbar -- this pops open the Volumes folder, so I can file anything on any drive with a simple drag and pop-up routine...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030715121724350