The information service is an important component of any Grid middleware as it helps users to find resources suitable for the execution of their applications. Traditional approaches based on data bases that have been designed for Grids of moderate size do not fit the requirements of wide-area large scale Grids composed of dozens of thousands highly dynamic nodes. In this paper, we propose a fully distributed information service relying on an unstructured overlay network. The core of this information service is the RW-OGS cached-based optimized random walk protocol. We describe experimental results showing that RWOGS outperforms state of the art protocols in terms of bandwidth consumption and resource discovery latency. Moreover, RW-OGS is optimized to increase the probability to discover nodes with free resources and to enable fast discovery of rare resources. The RW-OGS protocol has been implemented as part of the Vigne middleware aiming at easing the use of large scale desktop Grids for executing a wide range of distributed applications.