igraph library
A graph library written in C with optional interfaces to Python and R, developed jointly with Gábor Csárdi.
The Last.fm interactive artist map
An interactive layout of all (well, most of) the artists of Last.fm in 2D space, based on artist similarity relationships. Scrollable, zoomable, searchable, yay.
Spectral clustering of protein sequences
This was implemented as part of a research project that aims the application of spectral clustering to the analysis of protein sequences on a genome-wide scale. Qt, C++, ARPACK. It has a nice GUI and a command line interface as well.
Miscellaneous
See also my profile on GitHub for other miscellaneous projects.