I have been using this software for a while and I find it indespensable when flying in mountainous areas, or areas with dangerous approaches (Sion, Innsbrook for instance), I have flown along valleys in the dark with no problems. It has been a requirement of the FAA that all US registered aircraft carrying 6 or more passengers should have TAWS Radar fitted after March 2005. Follow the link to the Reality-XP site.
http://www.reality-xp.com/ I know it is Payware but I think it is worth the money.