Judging by the number of e-mails I get on this subject, a lot of folks don't understand the difference between computer and hi-fi speakers. For example, how is an Audioengine A5+ ($399 a pair) functionally different from a PSB Alpha B1 speaker ($300 a pair)?
Both have 5-inch woofers and tweeters, but the A5+ has something you won't see on the Alpha B1, or any hi-fi speaker: a volume control. That's because the A5+ is a "powered" speaker, meaning it has built-in 50-watt-per-channel stereo amplifiers. The Alpha doesn't have its own amps and has to be hooked up to a power amp or receiver. The differences don't end there: the A5+ can be used as a "nearfield" speaker on a desktop, where you might be just a few feet away from the speakers. The Alpha B1 was designed for stand- or wall-mounting, and listened to from 6 or more feet away. That's why it's classified as a hi-fi, not desktop, speaker.
Still confused? Let's break the speaker types into four categories: hi-fi stereo speakers, desktop/computer/media (nearfield) speakers, active (powered) speakers with built-in amps, and passive (nonpowered) speakers that must be used with a separate amp or receiver.
The hi-fi-versus-desktop definitions can be a little fuzzy, as some hi-fi speakers can be superb desktop speakers, but many fewer small desktop speakers have what it takes to fill a room with sound. Passive and active models are easier to define: if you already have a good amp or receiver, buy passive speakers; if you don't and want the most cost-effective speaker choice, buy active speakers.
So while the Alpha B1 is $99 less expensive than the A5+, you'll probably need to spend a lot more than $99 for an amp or receiver to play the B1. That's why powered speakers like the ones from Audioengine or Emotiva are terrific values, but powered speakers' sound is limited by the quality of their built-in amps. Don't get me wrong, they can sound great, but with nonpowered speakers you can get even better sound, it's just going to wind up costing a lot more money.
Some speakers like the self-powered Emotiva Airmotiv 5 or passive Harbeth P3ESR can do double duty, work on a desktop and fill a small room with sound. If you're not sure about a given speaker's nearfield/hi-fi abilities, contact the manufacturer.
The Mini Maggie ($1,495 a pair) is the best-sounding desktop system I've heard, but it can't fill a room with sound. These speakers sound best from no more than 4 or 5 feet away. They're not self-powered, and they need a great amp to really sing. There's no practical limit on an amp's quality that would be appropriate for these speakers, but Wadia's 151 digital amp ($800) is nice and small, making it the ideal starter amp for a Mini Maggie desktop system.
When shopping for small speakers, specify your needs. That way you'll get the best sound for your application. If still confused, ask questions in the Comments section.