There were 6 Coloured eggs and an Ice Key. You get them, go to some menu, and, because the N64 was capable of it RIGHT BEFORE BANJO KAZOOIE'S RELEASE, Pull the Cartridge out and Swap it with Banjo Tooie, therein unlocking things in each game.
You should prbably read up on it on Gamefaqs.
The Xbox Live Arcade version of BK has Stop N' Swop working with Nuts and Bolts, letting you unlock shitty vehicle parts in exchange for finding them. It's debatable as to how worthwhile this is.