My parents liked to invest in "educational toys" so we had one of these beauties, created by Texas Instruments in 1978. I don't remember much about it except that I could play a kind of hangman on it...I think it was called "mystery word." I guess the object was to improve spelling. There were also other versions called Speak & Math and Speak & Read. I know we had the math one too, and possibly even the reading version. Wikipedia has a pretty good article on this toy and it reminded me that an early version of the Speak & Spell was featured in the Spielberg film, E.T.
If you want to really relive this game, check out this link. It allows you to actually play with a Speak & Spell online.