Although I am sure there are others, LaunchBar is the first third-party Mac OS X app I’ve seen take advantage of the extensive Avenir Next family bundled with OS X Mavericks. (Apple’s Maps is the only other desktop app I know that uses it.)
As I’ve already enjoyed in Tweetbot on iOS (for a future post), Avenir makes a pretty good UI font. The experience feels fresh and usable. If you’d like, however, you can switch LaunchBar to the translucent “Yosemite” theme, which anticipates the next version of OS X and its system-wide use of Neue Helvetica.