Originally posted by: SirPsycho
I've heard good things about Rosetta Stone. It depends on what level you're taking right now, though. It's supposed to be good for vocabulary in particular, and perhaps not as good for grammar. They have demo versions available on their web site.
I can also personally recommend Pimsleur Comprehensive Spanish. It's not software, though; it's an audio-only course, but it was quite effective for me.