We've been using milter-greylist:
http://hcpnet.free.fr/milter-greylist/ ftp://ftp.espci.fr/pub/milter-greylist/
for almost a year now and it works great. I looked over a number of other options and I believe this was the only one that was a compiled executable. I think all the other ones I found were scripts and I wanted something faster.