Can anyone reccomend a good program (preferably free-ware) to experiment with a-life with? If you know of multiple such programs do you have a preference? Do you know which is the most widely respected? Is there any that have serious ones that have graphic displays and convenient interfaces?

I've been checking out Avida from the Cal Tech Digital Life Lab which seems really cool. I'm wondering what other programs may be out there though. I'm most interested in programs that simulate life at the micro as well as the macro level, and evolutiononary processes. I do understand abstraction like avida uses, i.e. I don't necessarily need a program that say has instructions that actually look like DNA, only intructions that can be formally similar in all the important respects--like Avida.