The DC Animated Universe, for those of you who don't know (which I expect is none of you), was a series of interconnected animated series and movies made in the 1990s and 2000s. They were aimed at children, but quickly proved very popular with adults because of the mature storytelling, great animation, and scope of the world. Needless to say, the DCAU as a whole is one of the most successful adaptations of DC Comics and its effect on the comics themselves has been immense. A lot of it has already been seen in the Super Posts for Batman: The Animated Series and Batman Beyond, so today we'll cover Superman: The Animated Series, Static Shock, and Justice league/Justice League Unlimited.
Mercy Graves first appeared in Superman: The Animated Series (Episode: "A Little Piece of Home", 1996) as Lex Luthor's valet/bodyguard. She first appeared in comics in Detective Comics #735 (August 1999) during the "No Man's Land" crossover event, and has since gone on to become CEO of LexCorp and to be revealed as an Amazon.
She made the jump back to television as the Smallville character Tess Mercer, a combination of Mercy Graves and the Superman: The Movie character Eve Teschmacher.
Livewire first appeared in Superman: The Animated Series in an episode of the same name (1997). She first began appearing in comics in Action Comics #835 (March 2006), but unlike her animated counterpart, she was born with her electric powers.
In addition to characters (most of which have already appeared in other posts), the DCAU also brought iconic looks to certain characters that were brought over to the comics, as well as a few other things:
Left: Supergirl as she appeared Post-Crisis (Matrix/Linda Danvers) (Superman #21, September 1988)
Middle: Supergirl as she appeared in Superman: The Animated Series (Kara In-Ze) (1998)
Right: Supergirl as she appeared after Matrix left (Linda Danvers) (Supergirl #51, December 2000)
Left: the way Parasite looked in comics in the early 90s.
Middle: how he was drawn in Superman: The Animated Series.
Right: His current look. The earliest appearance I've found is Superman #657 (December 2006)
Left: Toyman has he appeared in the 90s
Middle: Toyman has he appeared in Superman: The Animated Series (he's wearing a mask)
RIght: Toyman as he appeared beginning with Action Comics #837 (May 2006), before he was revealed as a robot and renamed "Toyboy".
Left: John Stewart's original costume (Green Lantern 1, June 1990)
Middle: John Stewart's costume on Justice League/Justice League Unlimited (2001)
Right: John Stewart's current costume (his eyes have also changed to green, but you can't see that) (Green Lantern #150, July 2002)
Power Ring III
Left: Power Ring II (JLA: Earth 2, 2000)
Middle: Green Lantern in Justice League Unlimited (2003)
Right: Power Ring III (JLA #107, December 2004)
The Javelin first appeared as the Justice League's main form of transportation in the Justice League animated series (2001). It made the jump to comics in Justice League of America #9 (July 2007).
John Stewart's military background