What is the most extreme weather found on another planet? On Venus, there is really inhospitable weather, as well as within the gas giants in our solar system. Are there examples of even more extreme weather on planets found in other solar systems than ours?
I'd say that HD 189733b is a good candidate for the most extreme known weather on another planet (outside our Solar System).

According to some recent news accounts, the atmospheric temperature is believed to be over 1000° C, with 7000 kph winds. (For comparison to the data in Rory Alsop's answer, that's about 1900 meters per second.)

And it rains molten glass. Sideways.

UPDATE : As Guillochon points out in a comment, HD 80606 b likely has even higher winds, though they're not continuous. It's a Jovian with an extremely eccentric orbit. Quoting the Wikipedia article:


  Computer models predict the planet heats up 555 °C (1,000 °F) in just
  a matter of hours triggering "shock wave storms" with winds that move
  faster than the speed of sound, at 3 miles per second.


which, in civilized units, is about 4800 meters/second. Probably no molten glass rain, though, so it's not clear that it's more "extreme".