Many, Apart from the above list, Zone Alarm Free if you are not in a mood to spend time over your program, Filseclab, Sygate are all good ones.
Kerio 2.1.5 or 2.1.4 (was more stable for me,just don't use remote administration that has a vulnerability in that version).RAM usage is about 2-4 MB ram,CPU usage is the lowest possible.