Someone posted a while back an nice little url where you entered a domain, it found out its associated ASN and showed you the providers it used (or at least the ones bgp4 advertised).

I've tried searching with no luck, anyone remember?

Are you talking about http://www.domainwhitepages.com/ ? I think that has the information you are looking for.

Is this it?

http://www.fixedorbit.com/search.htm

??

Possibly http://visualroute.visualware.com/ ??