A cliché (pr: "clee-shay") is a word (or more usually a phrase) which has become so commonly used that it's original meaning has become watered down, to the point that people say it without thinking about what it means.
The English language has many clichés, and many more phrases that are destined to become clichéd. Normally, it is considered bad use of language to use clichés, but when it comes to making punny jokes, they are very useful. A sizeable number of the punny punchlines at BadPuns.com, across most of the joke categories, are clichéd phrases that have been twisted in one way or another to produce a pun.
I was going to say that they have provided rich pickings.... but that would be a cliché.