The two-headed 15-inch iMac G4 “Hydra” was a special software development kit released by Apple in early 2002 in an effort to help lagging developers port that applications from Mac OS 9 to Mac OS X. Capable of booting both operating systems simultaneously the hydra offered increased performance in the form of Dual 800 MHz PowerPC 7450 CPUs and up to 2 GBs of RAM. It sold for $2,000 (Apple Pro Speakers sold separately.)

iMac G4 with dual 15-inch displays showing Mac OS 9 on the left and Mac OS X on the right.