This is not an entriely serious post, as I know that the Pi itself - with its low price point and almost unlimited flexibility - is its own selling point.
But historically, there have often been items of software -typically games, although not always the case - which were so good that people bought the hardware just to get access to the software.
For example, I would argue that one BBC model B application sold more Acorn DFS / Floppy disc drive kits than all of the other possible applications put together, and that would be the floppy disc drive edition of ELITE, which had extra content compared to the cassette based version.
Another (perhaps less obvious) example would be CUBASE (and MIDI sequencer software in general) on the Atari ST.
If there's a 'killer app' for the Raspberry Pi, what would it be? RaspBMC and the other alternative media player suites must be strong contenders - but can you nominate anything else?
Did you buy your Pi to run a particular application or item of software? Or did you just buy the Pi, then go looking for things to do with it?