I first heard this over BBC on the way to NIE. I am so glad its on PLOSBiology. Now at least it can be easily shared.
According to Breuer, Ndoundou-Hockemba and Fishlock (2005), "This paper documents what we believe to be the first two observations of tool use in wild western gorillas (Gorilla gorilla)."
"We first observed an adult female gorilla using a branch as a walking stick to test water deepness and to aid in her attempt to cross a pool of water at Mbeli Bai, a swampy forest clearing in northern Congo. In the second case we saw another adult female using a detached trunk from a small shrub as a stabilizer during food processing. She then used the trunk as a self-made bridge to cross a deep patch of swamp."
Reference: Breuer T, Ndoundou-Hockemba M, Fishlock V (2005). First Observation of Tool Use in Wild Gorillas. PLoS Biol 3(11): e380. Get that PDF here.