I used to take my Canon 5DmkII out in the rain all the time with no issues. That camera is weather-sealed, as are the two lenses I used with it. Canon L lenses are all weather-sealed, I think. I also had a Tokina 17-35mm ATX-Pro lens and it was also weather-sealed.
None of this stuff is totally waterproof like the Nikonos V that Phil has; that's a camera that can actually be used underwater.
There are several rain sleeves you can buy to cover cameras that aren't weather-sealed. I haven't tried any. All of them seem too big for the camera I use now, an Olympus Pen-F. Some of the Olympus lenses I have are weather-sealed but the body is not. I should have bought the OMD-EM1 mkII, it is weather-sealed, but it was $600 more than the Pen-F and I couldn't afford the extra at the time.