Who's This? The eyesore on page 2 in Who's Who vol. XXI.
How you could have heard of him: While he's recently been seen in Forever Evil and Trinity of Sin as a member of the Secret Society of Super-Villains, post-Crisis/pre-New52 appearances have been pretty limited and gone out of their way to be humiliating. In 2006, he's tortured and seemingly killed on video in an issue of Manhunter AND is seen as a drug-addled informant for Black Lightning in Justice League of America #1. His entire role in Final Crisis is getting arrested in his embarrassing costume. And in the Batman Beyond comics, he is found violently murdered in his apartment after he has paid his dues to society and retired. No respect. He also appears in a bar scene in the Brave and the Bold cartoon.
Example story: Detective Comics #466 (1976), "Signalman Steals the Spotlight!" by Len Wein, Ernie Chua and Vince Colletta
But this is perhaps not the most effective version of Batman ever written. When we catch up with Bruce Wayne, he's sitting down to watch baseball and WILD HORSES COULDN'T TEAR HIM AWAY. The bat-signal, however, just might. He'll catch a bit of the game anyway, because S-Man is robbing the stadium. He uses a gas gun to knock out the stadium staff and his special power to "signal" things with signs:
Who else? Oh we're staying with the obscure villains. The next one won't be so colorful though.