I recently, mainly through searching for other things, found out about a couple of cool Easter Eggs/bonuses in some older Sega racers that I thought were a bit on the obscure side and therefore worth sharing.
You can play as the bike from Super Hang-On and the F-14 from Afterburner in the arcade version of Power Drift by winning certain races. The go kart thing even has a nice little transformation animation as it turns into them.

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You can also play as the bike from Super Hang-On in the Megadrive version of Ayrton Senna's Super Monaco GP 2 by inputting a cheat code.

You can also use a cheat code to play as the car from Virtua Racing in the Japanese Megadrive version of Outrunners complete with a few bits of music and sound effects from VR as well.
Has anyone else come across any other cool Easter Eggy bonuses like this? I'm guessing stuff like the bonus Scud Race and Daytona 2 tracks in Outrun 2 are pretty well known about now. Another one I can think of off the top of my head is the ability to play music from a couple of other Sega games from the initial entry screen in the Saturn version of Daytona USA by inputting certain initials. There was also that DLC for the PS vita version of Ridge Racer that let you play as the Hornet from Daytona in it and of course the AGES car in Sonic Transformed.