There is Underhive which was released 7 years (?) or so after the original. You can download that for free from the Games Workshop site under the Specialist Games section.
You can also find alternative rulesets at places like EasternFringe.com and WarSeer.com which have really forums devoted to Necromunda. Lots of continual discussion on many aspects of our beloved game.
Our group is trying out Anthony Case's Community Edition rules which is a modified Underhive ruleset and contains many clarifications, changes and additions to the core rules. You can download that
here.
I'll also plug my own site I created to make life easier for players to manage their gangs and campaigns, it's got a lot more features now to do almost anything you want in terms of customization. The site is
Yakromunda