I have the JBL Creature II system consisting of two small stereo speakers (each about 1/1/2 inches in diameter) in 3"x3" enclosures and an approximately 4 inch diameter subwoofer in a 9"x9" enclosure. They look kind of like something that would come out of a UFO and I guess that's why they're called Creatures. The stereo speakers each plug into the subwoofer and the subwoofer plugs into the iMac's headphone jack.
Despite having such small speakers, the system produces a surprisingly large volume of really good sound. Unfortunately, it is no longer being produced although it still may be available. But the price (about $300.00) that some dealers are asking for is ridiculously high. It shouldn't cost much more than $100.00.
http://shopping.yahoo.com/p:Harman%20Kardon%20JBL%20Creature%20II%20Computer%20S peaker:1991385918
There are a couple JBL computer systems consisting of stereo speakers and a subwoofer that are still available and apparently are successors to the Creature system. One is the JBL Spyro 2.1.
http://www.amazon.com/JBL-Spyro-Speaker-system-White/dp/B000IS1ZYO/ref=sr12?ie=UTF8&s=toys-and-games&qid=1243287075&sr=8-2
If you can stand the purplish-red fuchsia color, you get get it for about $70.00 from Amazon. In white, it costs about $120.00 including shipping.
The other one is the JBL Spot 2.1.
http://www.amazon.com/JBL-Spot-2-1-Speaker-System/dp/B000M39268
It looks as if the subwoofers of both the Spot and Spyro systems are exactly alike. At $250.00 from Amazon, the Spot is way too expensive. But a few Amazon merchants who have very positive ratings carry it for a total of around $130.00, shipping included.
If the Spyro and Spot speakers are at all similar in performance to the Creatures, I believe that you'll end up with excellent audio, especially considering the very small size of the speakers. Just be sure that if you order either system and are not satisfied with it, you can return it for a full refund.