This is my opinion, which of course will change depending on whether you want 2.1s, surrounds (5.1+), digital input, and/or needing a home theater environment.
For 2.1s...
If you want great sound and something that can shake the ground, then go with the Klipsch Promedia series. Otherwise, Logitechs higher end speakers are not that far off behind Klipschs.
If you want mac-like looks, decent sound, and a small footprint, go with the HK soundsticks or the JBL Creature IIs.
beatle888: Rule number 1, don't buy speakers with an Apple store price tag. You can get far better speakers below $300. I also don't recommend a reciever/speaker setup unless you plan on truly taking advantage of the power and/or will be utilizing it in a home theater. Using powerful speakers that may take up half of your deskspace in front of an iMac just for mp3s/you tube doesn't make much sense to me. And computer speakers will never phase out real speakers.