Stay's not without its quirks. In particular, if you need a version that works with Mountain Lion's Spaces feature, you must buy the utility directly from Cordless Dog--the Mac App Store version doesn't support that functionality, due to Apple's rules about using private APIs. In addition, there are a few apps that aren't compatible with Stay (including Steam, X11, and most of the apps in Adobe's Creative Suite) and some apps (including Google Chrome and some versions of iTunes) require manual window linking. Finally, I've occasionally noticed that some windows don't restore correctly; however, another round of storing the window locations usually fixes that.
All in all, Stay is an invaluable tool for anybody who goes back and forth between display configurations, helping you avoid that hair-tearing, bewildered cry of "What the heck is that window doing there?"
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