Hello, South Park! Not only is it one of (if not still) the most dominating show on Comedy Central, but for a time, it was also right beside South Park on TBS for marathon/back to back showings (four episodes in a row!) weekday and weeknight.
CC later dialed back this practice in favor of a similar obsession with Futurama once they picked it up.
It seems that as of 2013, this treatment has long died down.
As of spring 2014, Disney XD in the UK still adores the show enough to run two hours' worth of episodes twice every day, run special seasons like "Dr. Doof's Worst Inators", and to promote new episodes heavily as a lead in to the UK premiere of Wander over Yonder.
On the subject of shows without a huge fanbase, most of the network's time not spent on ponies or Transformers is instead spent on Game Shows.
Now, The Hub seems to adore Animaniacs, airing promos for the show during every program on the network (even during Animaniacs itself!), and in addition to the Christmas marathon, they ran a two-hour one for Martin Luther King Day (along with The Super Hero Squad Show) and a two and a half-hour one for Super Bowl weekend. Normally, the show airs for three hours on weekdays.
For those with small children - Caillou on the Sprout network. These are shown in 3-episode blocks 15 minutes in length, and only the first three seasons. This means there are only 25 blocks. Two blocks are shown in the morning, one in the evening (repeated 3 times in the overnight segment), and a full hour is shown from 1-2PM (3 blocks with commercials in between). This means Caillou is shown 9 times a day. There is a 100% turnover of episodes TWICE A WEEK.
As of 2014 it now seems to be Sally Bollywood and Famous Five: On the Case. To wit; both shows air three times a day Monday through Friday (at 7/7:30 a.m., 6/6:30 p.m., and 9:30/10 p.m. respectively), and on Saturday & Sunday at 6/6:30, but Famous Five airs again on both those days at 10:00. Oh, and unlike before, where they'd air one episode then rerun it at a later time, it's three different episodes each day. Consider that both shows only have 26 episodes note Okay, actually Sally has another whole season, but qubo seems to refuse to air those episodes and they blow through both of them in about two weeks or less.
Before it got changed to Disney XD, Toon Disney would air a marathon called Pumbaa Bowl every year. They aired episodes of Timon & Pumbaa for the whole day.
Toon Disney once aired a whole day-long marathon of Yin Yang Yo that consisted of the same few episodes being played over and over, the entire day.
The UK channel seems to adore Sofia the First, and like airing the short-form series Nina Needs To Go! five times a day. The latter example is no surprise, as it was produced by Disney's United Kingdom branch.
Any Disney Junior network, regardless of country, will adore the short form series "The Doc Files"
The whole time Cookie Jar TV was on CBS, only one show stayed on it's lineup for a long time, Busytown Mysteries. They aired it for one hour every Saturday.
ABC lovedRecess, to the point where it was usually shown twice every week. Not that anyone minded; the show was very popular. Plus, Toon Disney would frequently show marathons of the show (Disney Channel would just show it twice a day) until 2007, when the show was moved up to 9:30 AM every weekday...when the target audience was in school. It remained like this even after the switch to Disney XD in 2009, and was removed in 2010 (and was briefly revived in 2011).
CBS adored Garfield and Friends back at the height of its popularity. After the first season, they showed it for an hour until it was finally cancelled in 1995. Actually, they originally were planning future seasons, but CBS wanted to pay less and less money. It managed to run a year longer than Muppet Babies, the previously Adored by the Network show.
When Teletoon Retro got the rights to Garfield and Friends, they showed it 3 times every Monday through Thursday. As of 2014, the show airs 6 times a day on weekdays.
Teletoon even held a marathon of the show on September 1, 2012, where they showed six episodes back to back from 3:00PM to 6:00PM.
The Latin American Tooncast channel also shows Garfield and Friends for an hour each day like Nick used to do. Also, just like InuYasha is for [adult swim], Garfield and Friends is one of Tooncast's network staples, as it has been airing since 2005. If you count its run on Cartoon Network, where it transferred from, that means it was actually aired for 20 years altogether in Latin America. Due to this, it's not going anywhere soon.
The Swedish version of Jetix and Totally Spies!. They literally had a programming block called "Totally Jetix": Only Totally Spies, all day, every day.
Though, the effects of these changes might be a test schedule for the network's upcoming rebrand in the fall.
Polish station MiniMini recently started broadcasting My Little Pony: Friendship Is Magic episodes of both Season 1 and 2 everyday, which amounts to 4 episodes per day from December 2012 to March 2013 (decreased to 3 episodes per day from mid-March 2013 to June 2013 and again to 2 episodes per day from June 2013.).
Fox Kids and Jetix broadcasted all-night marathons of spanish animation series as Memé y el seńor bobo, Mortadelo y Filemón or Iron Kid.
As the page quote indicates, King of the Hill was very popular on FX prior to the syndication rights lapsing. Marathons were common during the early/mid-'00s even without a holiday or other justification.
NBC had this problem with The Smurfs during the 80s, where it was run in 90-minute blocks during its heyday. Nevertheless, it was a huge ratings grab for the network, and it topped even some of the network's primetime shows.