A fast-paced surreal series where each episode is a parody of various anime conventions. At the beginning of each episode, the creator of the original manga (which was apparently still funny but did NOT do this type of thing; however, it was widely considered un-adaptable to anime at the time, so this is an Adaptation-esque attempt to adapt the unadaptable) giving his permission to turn his story into whichever style they're doing this week. (styles include: Animal Anime, Sports Anime, Magical Girl, GAINAX-style Deep And Serious Anime, American Animation, among many others) Then the show progresses in that particular style.
The plot of the show is an underground association is trying to take over the world; the only problem is that it's three members: Il Palazzo, the leader; Excel, his undying servant with a crush on him who also tends to fuck everything up; Hyatt, a beautiful space princess who has a nasty habit of dying all the time. Il Palazzo (all the main characters are named for hotels, see) will come up with a plan, typically in line with the genre (i.e., for the Sports One, it was for them to infiltrate a bowling alley to learn more about their future slaves). Also, Excel is always looking for food, as she's typically on the brink of starving to death every episode, and as such, she's got a "dog" (who looks suspiciously cat-like, but oh well) named Menchi, which is the "Emergency Food Supply".
There was a direct-to-video spin-off called Puni Puni Poemy, a dirty parody of Magical Girl, though the only characters in both series were Nabeshin (the real-life director of both Excel Saga and Puni Puni Poemy) and his wife.
I'm actually forgetting a whole bunch of characters (Nabeshin, The Will of the Cosmos, Pedro, the neighbors including the guy who only speaks in subtitles, the pedophilic scientist), but yes, you get the idea. It's more silly than things like FLCL and much more plot driven than Super Milk-Chan, and is, honestly, more geared to people who are already anime fans (as they'll get the various references), but it's still a pretty awesome series. It's 26 episodes long, although only 25 were ever aired, as the 26th episode went Too Far and the censors wouldn't allow it (I've never figured out whether or not the 26th episode was always intended to only be a video release; if they made it for broadcast and had it rejected; or, if they made it technically for broadcast with the intent that it'd be rejected.), so it only appears on home video/DVD releases of the series.
This one probably won't be screened on Adult Swim. Or anywhere, really. But it's really good and should be rented. Or purchased, like I did. It's awesome.