As far as interactive penis expansion goes, I think the flash game Nimin on furaffinity is pretty fantastic. It's been mentioned in the breast expansion thread, but it definitely deserves a place here. After all, check out how far I was able to get with my character:
You began your journey as a Human.7 feet and 4 inches tall, you wield prepubescent hips and a firm butt on an overall musclebound figure. You look much like a male horse, your face slightly longer than normal with a sort of muzzle and a bristly tail swishing upon your backside. Atop your head, you have large tear-drop shaped ears that flick every now and then, able to hear quite well, like that of a horse's.You are currently wearing a tattered shreds, covering your fur.Above your groin rests 3 bus-sized wangs. 3 hide within a fuzzy sheath that protrudes from your groin, around 25.6 inches in thickness.Beneath your cocks swing a scrotum filled with 7 beachball-sized testicles.
I'm looking to get even bigger. Here's the link, if you're interested and don't want to search the site:
http://www.furaffinity.net/view/6627968/