We've written a Firefox Extension that displays the AS Number of every website visited along with some additional interesting information. All data is updated daily and the prefix to AS number mapping is from a DFZ BGP feed.
You can download the extension here (a screenshot is available too):
http://www.asnumber.networx.ch
If you have any feedback, comments or feature requests please contact me either directly or at asnumber@networx.ch. We are working on providing a lot more information on the RIR IP address delegations but need fully access to all RIR's whois records for that. I've completed all paperwork last week and we should receive full access in the near future. Another thing I want to do is to show the number of RBL (Spamhaus, etc) listed IPs per AS.
BGP Experts, Traffic & Network Engineering Gurus