They're good value frames, which is why they're so popular. Very solid, pretty much bombproof. Lightweight? Not on your life. Surly makes their frames out of 4130 cro-moly steel. Website is here...

Another good steel frame manufacturer is Soma Fabrications. In my opinion, they're much sexier bikes, and made out of Reynolds 631, so I believe they're a bit lighter yet still incredibly well built, solid frames.