The physics arXiv blog from MIT Technology Review points to a paper that describes a way to use pulsars as an interstellar GPS system.
With the co-ordinate system established, any interplanetary spacecraft could then use the signals from these pulsars to determine its position in this co-ordinate system to within a few nanoseconds, which corresponds to about a metre.
Yes, I know, after the ant compass post, some of you are beginning to wonder about my sanity. Isn’t it interesting that “GPS system” has become the media synonym for basic navigation? I figure it’s all because of iPhone marketing.