does anybody know about any free speech tool software i can downlaod from internet ?

i have seen in some movies that u speak in microphone and computer understand ur speech and write it for you. i bought a new toshiba laptop soi have a speech tool software in it already but it can only understand certain commands but i m looking for one with which it can understand every text i say. so if anybody know about any software which i can buy or download from internet, please let me know.
thanks