Game Voice looks interesting, but requires hardware to use it.

I used Microsoft SAPI 5.1. It isn't too bad. The best thing is that anyone can download it for free.

I am currently working with Fonix. They have a very nice, light and fast system. They even have an SDK especially for games that offers wickedly fast and accurate recognition without a big hit on the CPU, and works well in noisy environments to boot. This isn't an off the shelf product, though. You'll need to contact them if you are interested in using it.