I've tried this a few times in DOD:S but when you get to the 3rd person they just slide off :(

I remember doing stuff like that in GoldenEye. It was ridiculous how many remote mines you could stick on top of another on a wall! We'd do that until they'd start disappearing (I want to say around 20, but it's been years). You'd see this stick of mines coming out of the wall. Then we'd shoot them with a couple of rocket launches.
Multi-player while dual wielding rockets was crazy.
On most shooters, me and my friends will sometimes have a knives only match. We usally end up cheating before it's over with, though.

Forge is possibly the best map making fun, crazy thing ever invented.
One of my friends and I created a map where there were two sides separated by a rather high wall.
It was possible to jump over the wall, but it took some timing to get it right.
On each side there were all types of grenades, so basically the rules were to throw grenades on to the other side of the wall to kill the other person.
Oh and invisibility was turned on so if someone decided to jump over to fight...
Chaos ensues.

One of the craziest maps I've played on was an old Half-Life one. The thing was a mess of rooms and boxes, but it had one of those trains in it which was on an invisible rail weaving through the sky. What made us lulz was the speed limiter had been removed, so it would glitch its way though the level at an impossibly fast speed (if someone got on and sped it up). The amount of times we'd get gibbed just attempting to jump back on to slow it down; never had so much fun at LAN.

I've played a whole lot of crazy Wolfenstein Enemy Territory maps.
- 1 in South Park
- 1 on a giant pool table
- 1 on a giant basketball field where you had to jump to the basket with a gold bar
And some more I can't remember.

I remember the multiplayer in Duke Nukem: Time To Kill was garbage, so my brother and I came up with several house rules to make it playable.
The most common was one we called SentiNeil*, where one player would collect all the weapons in the level, then stand on the highest ground available while the other player ran as far away as possible. They'd then have to rush towards the armed player, avoiding death however possible (without returning fire) and trying to climb up to the same level as them. This was quite fun.
Our other house rules weren't so much gametypes as scenes we'd repeatedly perform, with stuff like the player mistaking an armed pipe bomb for a 'remote controlled bone,' which would then blow them to bits.
Also, in Timesplitters 2 we'd always include the bot Calamari in our games, with the only real 'rule' being that we'd shout CALAMAAAAAAAARIIIIIIIIIIIIIII whenever he showed up.
edit: and I don't believe that house rules are a 'lost art' (though I do agree with the thing about mods/mutators stealing their thunder), I've had lots of bizarre metagames going on during games of TF2 or L4D... and if that sort of thing goes on between complete strangers, it's safe to assume blood relatives and friends inflict just as much madness upon the games they play as they ever have
*despite neither of us being called Neil, nor the game featuring a Neil, nor either of us knowing a Neil

hehehehehe... Indeed :D
Liero with Nukes only. Lots of weapons variations in Worms. Classic, knives-only games in CS, or Zombie-'MOD' for CS :D Bug-running in Half-life (oh that was fun)

Also, since nobody's mentioned it - Garry's Mod is another breeding ground for this sort of thing. It's a game that lives and breathes by the players' ability to improvise.
Here are a few short films compiling some of me and my friend's exploits in the game:
contraptions
travels in nihilon
so much troubles in my life
timeclock