In the Babylon 5 universe, Dust is an illegal drug that causes its user to have the normally dormant "telepath gene" temporarily activated. This allows them to poke around in other people's minds much like normal telepaths. The process is very traumatic for the person being violated, temporarily resulting in insanity, hallucinations and other psychotropic effects.  Mundanes usually recover from this experience within a day or two, but if a telepath is violated in this fashion, they almost never recover--they stay insane for the rest of their lives (it is never explained quite why this only occurs to telepaths).  

G'Kar used dust to enter Londo's mind once, in the third season episode Dust to Dust, and was incarcerated in the station's brig for a while as punishment. By poking around Londo's head, G'Kar was able to find out  about Mollari's connection to the Shadows and his true role in the corruption of the Centauri Republic.  Kosh also used G'Kar's altered state of mind to give him an artifical revelation concerning his role in things to come, resulting in G'Kar becoming more spiritual and philosophic as the series progressed forward.

We also find out that Dust was actually developed by the Psi Corps to try to turn mundanes into telepaths, but it failed miserably and did not produce one converted telepath of any appreciable strength.  The Corps keeps it on the market in the hopes that it will eventually work on someone.