Most speaker docks designed for use with iPods will work with any MP3 player. Instead of sitting in the dock they are typically connected via a 3.5mm headphone jack. The disadvantage with this is that it will not charge the MP3 player as it plays.
If you'll be using a docking speaker system with an iPhone look for one that has the Made for iPhone logo as these have been developed to Apple's specifications, so they're more likely to charge your device and make the best of your device's functions.
Different kinds of docking stations:
Transportable speaker docks: Ideal if you just want to be able to move your music from room to room in your house. A transportable system generally gives you better sound and louder volume than a smaller, more portable speaker and can still run off batteries.
Desktop speaker docks: Small enough to fit on a desk or dresser, these systems can offer good sound quality in a more compact package. Some have alarm-clock or radio features.
Portable speakers: These are the kind to buy if you want to be able to pack your speakers in your luggage and need something small and light-weight. They will not offer the kind of sound quality as a larger docking station and may attach to your device through a cable rather than a dock.
Docking station features:
Play around with the sound adjustment tools on your speakers - take your time, prerfecting audio calibration is a delicate operation. A quality system provides good balance between the treble (upper), mid range, and bass (lower) frequencies, producing full, rich sound while preserving detail.
Function to charge your device:
Many speaker docks charge your device while it is playing which is a useful function to have. Older speaker docks may not charge new iPod models (due to a change in the charging technology) so double this function if you specifically require it.
The more speakers a docking system has, the better and more well-rounded the sound quality will be. If your player boasts several speakers and a wide frequency response, the audio will be louder, more crisp, and generally more impressive.
You can connect some speaker docks to a computer using either a USB or a Firewire connection. This allows easy connection of the iPod to a desktop computer or laptop for updating music content without disconnecting it from the speaker dock.
Most speaker docks have magnetic shielding. If placed too close to a traditional TV, models without magnetic shielding can distort the picture.