It's actually really simple to create voice commands for yourself... you basically hard-code the voice sample... the easiest/cheapest way is to download an "oscilloscope" program for your computer that turns your microphone jack into an oscilloscope... then you can analyze the signal from various voice commands... you'd want to take multiple readings and find a range.. so that it matches every time..
But, i believe there are already things out there... hobby electronics is really huge...and companies design devices for those of us with this hobby.. you'd probably want something like...
http://www.parallax.com/Portals/0/Downloads/docs/prod/comm/30080-SayItModule-v1.1.pdfI've never used one, don't know how well they work.. or how expensive they are... but i think they are under 100$... but i've read about people using them for their microcontrollers (The most popular being Arduino)