If you're looking for a cheap SIM-only plan without a contract, iiNet has just launched themselves as a MVNO. With plans starting from $10 a month for $150 worth of calls and 200MB of data, the plans sound pretty good... although you do need to be an iiNet customer to jump on board.
There are three plans available for potential customers - the $10 a month plan mentioned above, plus a $20 a month plan including $400 worth of calls and 500MB of data and a $40 a month plan including $750 worth of calls and 1.5GB of data a month. All plans have a 35c flagfall with calls charged at 90c/minute in 30 second blocks, with extra data charged at 20c/MB. That may not be the best value out there, but given the included value in each plan, it's not too bad. Each plan also includes free access to Facebook, Twitter, eBay, LinkedIn, Foursquare and MySpace.
The other potential issue is that the service will be piggybacking on the Optus network, so if you know you have issues with Optus reception, this may not be the plan for you. Although given that there aren't any lock in contracts, you could try it and see how it works without spending too much money.