We don't use open source stuff here (much to my chagrin) and we work with larger scale dynamics (ocean basins and the like), but see the following links from a quick google:
http://gfs.sourceforge.net/
http://sourceforge.net/projects/duns/
http://www.opencfd.co.uk/openfoam/
There are some limitations amongst these (no unstructured meshes etc.) but worth a look anyway. As I mentioned, I have no personal experience with any of these, just putting links up for you to explore.
I had another one in my head, but I can't seem to find a link for it right now. If it occurs to me, I'll post it.
edit: I also found this seemingly quite comprehensive list:
http://www.cfd-online.com/Wiki/Codes
