I've been wanting a Teenage Engineering op-1 for years, but I can't justify paying $1300 for something I might not use a lot. But the Behringer TD-3 synthesizer, at about 1/10th of the price, looks like it's as much fun as the op-1.
It's a remake of the Roland TB-303 Bass Line that came out in 1981. According to Wikipedia, the TB-303 "was a commercial failure and was discontinued in 1984. However, cheap second-hand units were adopted by electronic musicians, and its 'squelching' or 'chirping' sound became a foundation of electronic dance music genres such as house and techno [this song being the one that started it all]. It has inspired numerous clones."