One of my favorite kinds of humor is that which results from plays on words, or from people misinterpreting or otherwise mangling perfectly normal words. So, here is a special category for such material. This will also include language gaffes, and anything else language- related that does not fall into another category.
For more fun, check out Engrish.com, my all-time favorite language humor website. If you're a fan of English-language humor, you should also check out the "Anguished English" books by Richard Lederer; several of the hilarious bits on this page are taken from Mr. Lederer's collections.