NickToons (sister channel to Nickelodeon and TeenNick)
(launched as Nicktoons TV in 2002; rebranded as Nicktoons in 2003, as Nicktoons Network in 2005, and as NickToons in 2009)
Nickipedia; official website; TV Tropes; Wikia; Wikipedia

I mostly think of this as a network for reruns of Nickelodeon cartoons, but it has aired some things that Nick didn't, or that aired here before Nick, or concurrent with Nick, or... whatever.

Martin Mystery (7/4/2005 - 4/27/2008; originally aired on M6 & Canal J in France, YTV and Vrak in Canada; some episodes aired on Nickelodeon prior to Nicktoons)
Skyland (11/26/2005 - 5/25/2007; also aired on France 2, in France, and Teletoon, in Canada; I think I only saw s1)
Kappa Mikey (2/25/2006 - 9/20/2008)
Invader Zim (6/10/2006 - 8/19/2006; s1 aired on Nickelodeon; I don't think I saw any of s2)
My Life as a Teenage Robot (10/4/2008 - 5/2/2009; s1-2 aired on Nickelodeon; I don't think I saw any of s3)
Wolverine and the X-Men (1/23/2009 - 11/29/2009)
Iron Man: Armored Adventures (4/24/2009 - 7/25/2012; I only saw s1)
The Troop (10/29/2012 - 5/8/2013 [s2e8-e14]; s1-2e7 aired on Nickelodeon; I didn't see any of s2)
The Fairly OddParents (2017; at least part of s10 aired on this network rather than Nickelodeon, which aired most of the series)