Paying for parking is irritating enough without having to dig up suitable coins. Parkmobile lets you pay automatically via a mobile phone app, and even sends you a reminder 15 minutes before your spot is about to expire.
The main limitation is that this only becomes useful if your local council supports the app. Right now, it's functional in Clontarf and Shelley Beach in Sydney, and Carlton in Melbourne, but the developers say expansion to other locations is planned. In the US, where the service was originally developed, some areas add an extra transaction charge; fingers crossed that won't happen here.
Parkmobile is a free download for iOS and Android. A BlackBerry version is also under development. If you're just keen to find the cheapest spot possible, check out the Cheap Parking app for Android or iOS.