Martin Dempsey, who started out making an early Exim implementation of greylisting, has also made one that should work with Qmail. You can find it at his greylisting page: www.digitaleveryware.com/projects/greylisting/
Gunther Mair has created a more sophisticated solution for Qmail. It is based on a postgresSQL database and is written in C. It can be acquired from: qgreylist-pgsql.sourceforge.net/
Greylite is a greylisting implementation for Qmail. It uses a so-called modified greylisting technology, which aims to make greylisting more transparent to MTAs and to make it more efficient. More information: www.greylite.net/