Webalizer_ASN v1.2b is available since a while, and according to my tests rather stable now. The ASN Data for Webalizer_ASN is based on Netlantis backend BGP engine (someone asked recently about Netlantis). If you experience a rainy lazy sunday, this might be a tool to play with a bit and build your personal Top-Ten ASN list.
Debian Woody users could use the precompiled package.
If you have existing orignal Webalizer statistics, you need to move webalizer.current file to webalizer.current.old before running the webalizer_asn - and please note: no warranty, you use the tool on your own risk - so have your statistics backuped first.