In the song "Empty Spaces" (or "What Shall We Do Now") on Pink Floyd's The Wall, there is a good example of backwards masking. From about 1:28–1:14 (just before the last verse of the song), you can hear Roger Waters very clearly (but quietly) saying, "Congratulations. You have just discovered the secret message. Please send your answer to old Pink, care of the funny farm, Chalfont." As he says "Chalfont," a voice in the background calls out "Roger, Caroline's on the phone."
On the vinyl, it's only in the right channel, but on my copy of the CD it's in both. Caroline was Waters' wife at the time. There is a Chalfont St. Giles and a Chalfont St. Peter in Buckinghamshire, but I don't know if there's a funny farm in either.
Many Floydians consider this a reference to Syd Barrett: "Pink" is the name of the main character in The Wall, and even though Waters says the story is semi-autobiographical, many fans see Pink as a symbol for Syd because he goes crazy in the end. The "funny farm" bit helps, too.
Roger Waters' album Amused to Death has another good example of backward masking, in the beginning of the third track, "Perfect Sense (part one)." The first minute and 50 seconds have a very slow, drawn-out message: "However (1:23), in the light and visions of the issues of Stanley (1:08–1:03), we changed our minds (0:51–0:50). We have decided to include a backward message (0:45–0:39). Stanley, for you, and for all the other, book burners (0:27-0:14). [Incomprehensible, angry screaming] 0:05–0:00)."
Rumor has it Waters wanted to use a couple clips from 2001: A Space Oddysey, notably HAL's voice and the heavy breathing, but Kubrick refused (supposedly because Pink Floyd refused to let him use music from Atom Heart Mother in A Clockwork Orange). After Kubrick's death, Waters used the clips in live performances of the song.