Anti-spam, anti-virus and content filtering ($500-upwards)
Competitive features like those in Gibraltar (http://www.gibraltar.at/content/blogcategory/4/32/lang,en/) and Endian (http://www.endian.com/en/community/about/features/) but those are based on Linux. I trust security and maturity of BSD any day.
The scan engines used for above mentioned requirements should be of reputable vendor with punctual & accurate updates. Content filter should be based on advanced content recognition by keyword beside the usual blacklist method.
Web url & content filter's and antivirus filter based on squid(or any proxy) must work by ICAP technology.
In this time free projects on beginner state. I found many commercial, and low free projects (poesia for example - supported Euro-Union).
AND squid-ICAP-client now not stable :(