I'll claim responsibility for starting this one, since the guy Dorian credited posted his two hours after mine.
Edit: Looks like I even beat the first altered banner on the Newsarama discussion which kicked it all off, not to toot my own horn or anything. Lots of fun ones in there, including the Speedball and Stupid ones that I'd wished I'd thought of first.
To that end, I'll post links to all the ones I find. If you guys would like to tell me at email@example.com, I'll get the links up more quickly.
Rann, Pantha's Head, DC
Her, U-Man, Dr. Bong, Inbetweener, Midget, Howard the Duck, Frigga, Pip the Troll, It, Gardener, Cancer, Angar the Screamer, Awesome Android
Groo/Not Reading It
A Whole Bunch
Hawkeye, Internet Jesus
Roger & Jessica Rabbit/Attractive Redhead, Gay Pimp, Lex Luthor/Bryan Singer
Ding Dong Daddy
The Question, the Spectre
Josie and the Pussycats, Them, Ambush Bug, Mr. Miracle
Wolff and Byrd, Counselors of the Macabre