that I've used so far is Skyvi. Not as mature a product but not bad. It talks in similar fashion as Siri albeit using a different voice. And it seems capable of learning.
Google Voice Actions. Just speak in very technical terms.
S-Voice on the Galaxy S3
sent from my Galaxy S2
Do any of these alternatives let you do the "remind me" thing? That's really the only thing I thought useful with Siri.