Most of this is hardly substantial, and I am sure to forget something, but here it goes:
wxPropertyGrid: various compile fixes that ended up in releases 1.2.8 (2007) and 1.2.12 (2008)
wxWidgets: GTK+ font handling fixes (2007)
SWIG: ticket 146 for a more helpful error message when resolving an overloaded function fails (2006)
S-Lang: patches to slparse.c (2007, change 86 to release 2.0.7) and slcurses.c (2006, change 41 to release 2.0.6); QNX port (2001, change 1 to release 1.4.4)
JED: QNX port (2001, change 8 to release B0.99-13)
rxvt: minor patches 907, 1213, and 1253 (2000)
Whereas the list above contains mainly fixes, and little in terms of added functionality, here is a project to which I actually did contribute significant pieces of code, adding features like spline interpolation and a Python interface.