If you've got speakers with a standard headphone plug, you can plug those directly into the iPod like you would headphones.
Alternatively, a bevvy of different companies make special iPod speakers. Most are made for regular iPods, but there are some good mini ones. Some of the best are:
Altec Lansing InMotion iM3 - Small and portable.
Altec Lansing InMotion iMmini - iPod mini version of the above.
JBL OnStage - Music Edition - Strong bass and a wide stereo field apparently.
Bose SoundDock - The most expensive, but also considered one of, if not the best.
All of these are available at the Apple Store Online, and various other stores, online and offline.
Alternatively, a bevvy of different companies make special iPod speakers. Most are made for regular iPods, but there are some good mini ones. Some of the best are:
Altec Lansing InMotion iM3 - Small and portable.
Altec Lansing InMotion iMmini - iPod mini version of the above.
JBL OnStage - Music Edition - Strong bass and a wide stereo field apparently.
Bose SoundDock - The most expensive, but also considered one of, if not the best.
All of these are available at the Apple Store Online, and various other stores, online and offline.