Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
This category of purely under-the-hood work is not immediately obvious and thus is often under-appreciated, but yields many useful improvements to the Free Desktop.
One notable entry here is automatic code parallelization in GCC (the main compiler on Linux) through Graphite, which should lead to significant performance improvements for all programs on computers with multiple processing cores. Of course, the scope of what can be done is limited due to this sort of conversion requiring a higher-level view of what a block of code is meant to do, but nevertheless this is very much a welcome feature that will keep our beloved compiler in line with the current state of compiler technology.
Filesystems have always been an area of interest for the Free desktop; from the plain-spoken and default ext3 to the fast ReiserFS and SGI XFS, Linux especially has always had a variety of powerful filesystems available for any task.