Round-the-world fares can represent one of the cheaper ways to fly, especially if you want to make a couple of stops on different continents, but many comparison tools (and airline sites) do a poor job of demonstrating what's on offer. The prosaically-named Round-The-World Fare Comparison Tool lets you see which major options are available.
While the site offers a lot of different fare types from all the major carrier groups, it does have one big disadvantage from an Australian perspective: it only examines routes from Sydney. While Sydney does attract a large chunk of incoming international flights, many of the cheaper fares do end up routing through Brisbane or Perth. So it's more useful in suggesting a likely price (and perhaps identifying fares you wouldn't have thought of) than as an actual booking tool.
Round the world trips represent one of the few occasions where I suspect a travel agent is useful anyway; the last time I was contemplating one, my agent actually found point-to-point fares that totalled considerably less than the round-the-world option. However, having some research data before you hit the agent is always helpful. If you've got other favourite sites for checking round-the-world fares, share them in the comments.