The first ever add on.

[Bomb Kirby]

The first v0002 add-on was Backyard.

[Ad Bot]

[Corbiere]

Does Blockland count as an addon? It's an addon to everything torquelike...

[frogger3140]

Does Blockland count as an addon? It's an addon to everything torquelike...

Nu

[Horizon]

A cookie

[oleg662]

[]

Going on-topic: I find it quite obvious one of the first Add-Ons ever, in Vanilla and Retail is a script.