In-car GPS. The iPod touch lacks a GPS receiver, but you can take advantage of several excellent navigation programs available for the iPhone by using an external GPS receiver such as the XGPS150 Universal Bluetooth GPS Receiver recently introduced by Dual Electronics. Note that traffic detection and other features may not work since they require a cellular connection, as does the GPS app by Telenav.
Landline phone. The main difference between an iPod touch and an iPhone is the cellular connection, which means there is no built-in way to make phone calls. However, the iPhone can make calls over Wi-Fi at home using Skype, Google Voice, and other apps. These apps may include charges for calling people who aren't using the app -- especially internationally.
Other. The iPod touch can act as a calculator, voice recorder, and many other devices. There's at least one exception, though. Unless you're in an area blanketed with Wi-Fi, an iPod touch can't serve as a substitute for an outdoor FRS radio, such as those from Motorola. An iPhone can use an app called HeyTell.
All in all, the iPod touch stacks up pretty well against the iPhone – and, of course, it doesn't require a cellular contract. A few accessories are needed for some tasks, but then again the iPhone needs them for some tasks, too.