Mac users have the same two solutions for internet connection sharing as Windows users - software solutions and broadband routers. The easiest, most reliable and recommended solution is always a broadband router.  When choosing a broadband router for a Macintosh network, make sure the router is configurable from a web interface.  A few broadband routers only come with windows applications that can configure them

When it comes to software based internet connection sharing on the Macintosh, there does not appear to be many options. Here are the ones we found.

IPNetRouter - NAT with inbound port mapping

http://www.sustworks.com/site/prod_ipr_overview.html

Surf Doubler by VicomSoft - NAT based solution

JProxyma - proxy server written in Java