When my friends and I first started playing MtG we came up with the following cards, most being inspired by my first entry:
Chronic Vale (aka 'Vale of the Purple Nuggets' aka 'Stoner's Vale')
Legendary Land
T: Add one mana of any color to your mana pool. If Chronic Vale is tapped for mana, take a hit from target bong; opponent gains control of Chronic Vale at the end of the turn.
"Dude, this Richard Kane Ferguson art is so fucking awesome..."After tapping this land for Blue mana a few times, I came up with this one after activating a Jester's Cap:
Jester's PawnshopArtifact
casting cost: 3
3, T: Sacrifice Jester's Pawnshop to search target player's library. If you find a card you want, you may make a trade offer for the card using cards you own from outside the current game. If that player accepts the trade, exchange ownership of the cards permanently, and you may shuffle your new card into your library.
It's a little wordy and the templating could use some work, but you get the idea.
The next was inspired by several friends' love of Frank Zappa:
Balduvian PhilistineHuman Barbarian
casting cost: UW
power/toughness: 2/2
U, T: Replace target cd that sucks in nearby cd player with one that doesn't suck.
"I mean, sure, Jazz from Hell
was pretty good, but what the hell is this crap?Ashnod's HoneyBucket*
Artifact
casting cost: 5
4, T: Sacrifice Ashnod's HoneyBucket to defecate on target player's library.
"Well now you know how I feel about 'Control Magic'."It combos nicely with [card]Goblin Welder[/card] in a multiplayer setting, if you have strong colo-rectal control.
Gamer's RevengeEnchantment
casting cost: 2U
Bob is such a flaming asshole.
This would be considered 'high concept' by today's standards.

'Gamer's Revenge' was actually a part of a cycle of cards that were based on real world observations. Other gems in the cycle included cards with the following 'rules text':
"Mike's sister is really hot and I even find her lisp cute when I'm drunk," to
"I honestly doubt Chris will ever lose his virginity," and
"This is the best damn donut I've ever tasted!"This flavor of geek humor no longer appeals to me, but I felt compelled to share.

Who else has come up with cards like these?
* HoneyBucket is a brand of porta-potty here in the Pacific Northwest (and elsewhere?)