Accessories can definitely enhance the user experience, usability and flexibility of a device. Some iPod accessories that fit into this category include:

iPod-auto connectivity- Allows users to access their entire iPod library, including using the steering wheel buttons to skip ahead, pause, or rewind.

Dockable, portable speakers- Altec Lansing seems to be a favorite. Integrates all the iPod's features, such as the alarm clock, remote control capability in some models, etc in a compact device.

iPod Clothing- Ski jackets, backpacks, etc. Contain buttons built-in to the clothing that allows the users to skip, play, pause, etc without having to get their iPod out or, in the case of the ski jackets, remove gloves.

iPod Remotes- allows a user to play, pause, skip, etc while an iPod is, say, connected to the stereo without getting up.

Cases- protection, portability, etc.

earbud-integrated lanyards- Users can snap the lanyard into the dock port, put it around their neck, and insert the connected to the lanyard without as much issue with tangled earbuds.

Armbands- Useful when working out

Combination car charger/FM transmitter- consolidates two functions in one, minimizing dongles and taking advantage of the power supplied by the car for the transmitter

Media Readers- allows users to upload digital camera pics from their memory cards to regular-size iPods

Voice recorders- self-explanatory

External Battery Packs- Useful for an additional 24 hours or so of emergency power for the iPod

Thumb-sized speakers- Tiny speakers, a little smaller than a shuffle for sharing your music with friends

Cassette adapters built for the iPod- allows steering wheel navigation when playing the iPod from the car's tape deck

All these allow the iPod to be used easier or in more activities.
easier: car integration, remote control, earbud lanyard, etc
more activities: clothing (winter sports, walking/hiking w/ daypack), armbands (working out), cases (walking), voice recorders (preserving thoughts, reminders, lectures, dialogues, phone calls), media readers (media dump to recycle media cards), speakers (filling a room with music, waking up to the iPod, falling asleep to it), remotes (stereo integration), etc

A good list of iPod accessories with reviews can be found here.