Several uses come to mind. None of these are supercomputer uses but rather cluster computing.
Distributed compilation. If you want to build non-trivial software packages (e.g. Firefox), it will take hours, even on a powerful x64 computer. A compiler farm of 4-8 RPi boards running distcc will cost much less than a top computer and consume less electricity, while providing comparable build times.
Media encoding. You may want to keep your media collection in high quality formats (FLAC, Blueray, etc.) which may not be compatible with some of the devices you own. With a media encoding cluster (e.g. x264farm), you could quickly convert selected media files to the format of your choice.
Network load distribution. If you have ever tried to run OwnCloud or similar server software, you probably know the performance on RPi isn't stellar. If you host a website visited by several simultaneous users all the time, you could use nginx or haproxy to distribute load between several RPi boards to increase performance (as a side note, you'd probably be much better off simply paying for hosting).