http://www.cs.wustl.edu/~schmidt/ACE-overview.html
Read the overview PDF.
http://www.cs.wustl.edu/~schmidt/ACE-papers.html
ACE has a very liberal license and lots of nice cross-platform abstractions concerning threads, communications, file i/o, etc.
I also have a question about SDL, does it require the whole screen (like most games?) or can it run inside a standard Win32/MacOSX/X11 window alongside other standard applications? (Please add your answer
to the FAQ!)
Mark