Ever since I discovered OSBF-lua, http://osbf-lua.luaforge.net/, I was pretty pretty impressed with how fast it learnt and how low maintenance it was once it was setup. It did a pretty good job and it probably doesnt need updating much (in terms of the software). Recently I've went back to trying out CRM114, the stock version isn't (well, it wasn't) 64bit clean and had some problems with 64bit machines/OS's. So I went straight to trying out http://hebbut.net/Public.Offerings/crm114.html

Admittadly the documentation and setup of the CRM114 spamfilters were abit messier to setup than OSBF-lua, but the sheer options and different types of statistical filters that it offers compared to OSBF-lua is just fascinating.