I think we won’t be able to help you very much without further information about what makes you “not satisfied with it”.
You need to provide us some insights about your scenario, what you need to have and current difficulties…
I also have deployed ISA Server in several of my costumers some years now and it is a very good and reliable (Layer 7) Proxy/Firewall. Of course this is always
For free, you may try Untangle, PFence, Smoothwall or Monowall.
Once more, depending on what you need/want you might not be able to replicate everything you can do with ISA Server in any other of my suggestions.