The Tango Bar might look right at home underneath an iMac, and it might come from XtremeMac, a company with "Mac" in its name, but the 10-watt amplified speaker will work with any computer that has a USB port.
The speaker is powered by USB, and also works as a USB audio device, which means a single cable for everything. Inside there are six speakers, and at the side there's a backlit volume knob that will annoy as it glows at night.
If you want to hook it up to non-USB device, you can. Just plug a USB charger into the speaker to power it, and then attach your phone/tablet/minidisc via 3.5mm jack. There's also a headphone port and a sub-out connector if you need some more oomph.
The iMac's speakers aren't bad, and perfectly good for a little YouTubing or even a movie. If you want to crank the tunes, though, an external setup is a must. And the Tango Bar at least matches the iMac's minimal lines. The Tango Bar is available now, for $100.
Tango Bar product page [XtremeMac. Thanks, Valerie!]