Never knew this existed and this is awesome. I googled the mattmakesgames forums and found the page that outlines its history.
I remember there being several Jumper fangames that were really cool. Are they all lost forever since that forum is gone? I'd love to go back and play those again. One fangame I distinctly remember is Jumper 2: Redux, and I think it's one of the most difficult games I played in my life. It was a level above Matt's games in difficulty.
There was a someone on the forum who was a good friend as well. We discussed An Untitled Story with each other a bunch. I remember how he grinded the Insanity run and eventually did it and blew my mind. BlackCastle on Insanity seemed like such an impossible feat. He may or may not be the user AUS in this forum. If you're around, AUS, please chime in.
It's incredible to see how big Matt is these days and I'm really happy for him. Pretty crazy, having grown up with Jumper and now seeing the behemoth that is Celeste.