I am using Bulletproof on WindowsXP. I don't really like it as the throttling doesn't really work and there is no overall speedlimit setting. I have tried various different programs and this is the one I stuck with.
Ive been using vsftpd on linux for quite a while now its an excellent program, and very secure if setup correctly. Probably the best ftp server for linux but also probably one of the more difficult to setup (If user authentication is needed). Anonymous ftp works out of the box. I had it running 24/7 for 4 or 5 months straight without a restart or glitch. I have since had to reboot for a kernel upgrade but it still runs great. (stability is probably more due to linux then vsftp)