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Probably the best is Speaktoit Assistant. I've had it on my table for quite a while now, and it works pretty well. The developers are also pretty good about updating it to add new features and refinements. There is also Skyvi, which is quite popular, but I've been pleased enough with Assistant that I haven't really given Skyvu a fair trial. As to whether Speaktoit Assistant works as well as Siri, I would say yes for the most part, although it is more demanding that you use the proper phrase to do things. Assistant does have some cool features like "Daily Briefing" that runs through weather, news headlines and your calendar for the day.
If you had a new Samsung Galaxy S3 you could use their built in "S-Voice" app. I haven't tried it myself. Also, don't forget that there are quite a few voice commands baked into Android, so you can do things like voice search and dictate right out of the box.