Found this Flashgame ... Its Portal havn't played it yet but you said you can add new games.. Here's the link- http://www.newgrounds.com/portal/view/404612 http://portal.wecreatestuff.com/
This is epic, it was released before Portal came out I think, I still remember playing it..some levels were quite hard...Would be a great addition to the arcade part! btw, for those got portal(if you don't, go fetch it from steam, since it's still free!) you should try this mappack too! http://portalmaps.wecreatestuff.com/ which is basically a remake of the flash version!