It might not be as complete as Gmane perhaps (I didn't stop to compare them on a feature-by-feature basis), but I have been happily using http://markmail.org/ for a while as a gateway to many mailing lists.
Markmail is great. If only they could replace their Flash-based graph by some sort of <canvas>. Also, I think Gmane has more coverage in terms of number of mailing lists.
