Squaresoft for having made Chrono Trigger, which, when I played it, very much did change my life. I was like 10 or 11 when I played it for the first time and it was like I fell in love. I drew pictures of it in school, went to certain parts of the game to listen to the music for fun (and recorded them with a little tape recorder so I could listen to it whenever). If I got depressed about silly things at school, coming home and playing Chrono Trigger in my room with a blanket around me was always comforting, long after I had beaten it. Sometimes even know I'll play through some (or all) of it just cuz, and it always makes me feel so damn good.
Nintendo for having made Legend of Zelda- A Link to the Past. Pretty similar to Chrono Trigger excpet I was 8ish when I played it, and my brain and personality was less developed. The "life changing" effect of it wasn't as intense as with Chrono Trigger, due to a more simple story and less engaging characters, but I explored that world over and over again, looking for secrets I hadn't yet found, and since I didn't have Gamefaqs or a strategy guide, it was all up to me to find all the secrets. It was a game that lasted a couple years before I had finally found everything.
Bungie for Halo. The Christmas I got an Xbox and Halo was the last real Christmas I had, as far as the kid version of that season goes. I was like, 16 or 17, and I really expected absolutely nothing, but I had mentioned to my dad that I thought Halo looked really cool. I honestly don't know why he decided to do it, but on Christmas morning I opened up an Xbox and Halo. My friends had all gotten Gamecubes, and they were strictly against PS2 and Xbox at the time (they now all own a PS2, and half of them Xbox's as well). Their game of choice was Rogue Squadron and Super Monkey Ball. They came over that night for my annual Christmas party, and brought all their new games. I was the only one with an Xbox or Halo... and over the course of the night, all my friends seemed to get more and more upset about having recieved a Gamecube, after seeing and playing Halo. The next day we went and bought extra controllers and played Halo for the rest of winter break, and within a couple months they all had Xbox's and Halo as well, and we started doing Halo themed LAN parties. Halo wasn't particularly life changing, but as I got older, that magical feeling I got from games as a kid wore off more and more, and Halo was the last true taste I've had of that.
One last one- Bethesda, for Morrowind. I played it for about 3 weeks straight, with no pauses in between. My aunt and uncle who I lived with at the time had an intervention with me about it. I felt detached from reality and I loved it.