Can I use USB 1.1 to charge or sync the iPod?

For most models, yes. Although it was not officially supported by Apple until October 12, 2005, you can charge and sync iPods that support USB 2.0 by USB 1.1 as well -- with the exception of the iPod touch models, which only support USB 2.0. However, USB 1.1 is substantially slower than USB 2.0, and consequently charging or synchronizing an iPod using USB 1.1 will be substantially slower as well.

Also see: Which iPods can sync and charge via USB? Which iPods can sync and charge via Firewire?

Can I use Firewire to charge or sync the iPod nano, iPod 5th Gen, or subsequent iPod models?

Unfortunately, iPod models starting with the iPod nano and iPod 5th Gen shipped solely with a USB 2.0 cable, somewhat alienating Mac users who only had USB 1.1 and Firewire ports.

The first, second, and third generation iPod nano as well as the iPod 5th Gen and iPod 6th Gen/classic models can charge via Firewire, but not sync. Subsequent iPod nano models do not support Firewire at all and the iPod touch and iPod shuffle models do not support Firewire either.

Also see: Which iPods can sync and charge via USB? Which iPods can sync and charge via Firewire?

Can I directly connect two iPods to each other and transfer music?

Apple provides no support for such a feature. However, there is a product called Hitch (now discontinued) that allows you to connect two iPods to each other as well as other MP3 players or digital cameras. The company provided a FAQ that might also be of interest.

Does the iPod support multiple voltages? Are adapters required?

The AC adapter that shipped with earlier iPod models supports multiple voltages and switches automatically between them, so a voltage converter is not required for traveling outside of one's "home" country. An AC adapter did not ship standard starting with the iPod mini 2nd Gen, and one does not ship standard with any current models. However, an AC adapter can be purchased separately.

Depending on where you are traveling, adapter plugs may be required. An inexpensive set of worldwide adapter plugs can be purchased from just about any discount or department store, travel store, or airport. Apple and Apple Authorized Resellers, like site sponsor Adorama, also sell a World Travel Adapter Kit, which is more expensive than a generic set of adapter plugs, but is elegant, extremely well-designed, and well worth the extra money.