Third Party Software for SpamAssassin
Open Source
Web User Interfaces for Administration or per-user configuration
Open Source Solutions that integrate SpamAssassin in some way
Central integration of sa-learn with IMAP Shared Folders
Commercial Services / Software
Spam Filtering Services offered by vendors using SpamAssassin
Connection Management Systems that add high performance blocking and traffic shaping in front of SpamAssassin
Network Appliances that use SpamAssassin
Maybe - Might Use SpamAssassin, we're not sure