I remember three of us lugging our PCs and big heavy CRT monitors to our friends house where we spent 12 hours playing through the original Rainbow Six. I think it had lan co-op, if not it was at least the whole night in multiplayer.

On the opposite side of the scale, I once connected my 486DX 33 to my friends Pentium to play Descent over a serial connection.

As his was the fastest machine, the game played normal for him. All I saw was a game updating the screen once every 5 seconds. That wasn't much fun. 😄

Playing in the computer lab in highschool at lunchtime was fun. Back before the school networks blocked that kind of thing

Getting my friends over for the weekend and playing borderlands and doing the glitch to loot general Knox's Armory with no time limit over and over then fighting over all the loot.

Or when I was quite young having my friend sleep over and then pretend to sleep and once my mum went to bed we would play medal of honor rising sun split screen on a tiny 10" CRT. We would put a blanket over us to conceal the light and sit with our faces right up to the screen. We would stay up all night playing and my parents would get so pissed when they caught us I'd lose my ps2 for weeks but it was worth it.

Big halo matches and custom game modes. Flying out to areas normally out of reach on maps in Halo 2 using a beam sword exploit.