spec me a rain cover

[LionOfJudah]

Hi Guys,
I have a Nikon D90 and am interested in a rain cover/protection for the camera. Can anyone recommend me one? and also are they worth it and do they affect picture quality in any way?
Thanks

 

[LionOfJudah]

at the moment i have kit lens 18-105 but thinking of getting possibly a 70-300 lens.