A couple memories spring to mind. None of them are "last stands" per se, but close enough I think.
One was Halo: Reach. It was a 4v4 team deathmatch, where immediately the three on my team (I think they were a party) and one on the other team left. So it was 1v3 in their favor, but these guys weren't the best team players. They spread out for most of the game, allowing me to take them on individually and gain the upper hand. I was 4-5 kills ahead when they figured this out and started grouping up. At this point they took back the lead by a single kill with 10 seconds remaining. I starting sprinting around, trying to find someone to tie the game with. I found one and was able to take him out with him only bringing down my shields but one of his buddies ambushed me and killed me just before the game ended. Because of that, I lost by a single kill, and the guy who got me promptly began teabagging my corpse as was expected. As soon as he did though, his teammate came over and smacked him to kill him. He then looked at my body, bowed his head, and walked away. Maybe he just hated teabaggers and I'm reading too much into this, but I like to think he just didn't want to disrespect me after that game, and a teabagging was not the way to see me go.
Another one back in the L4D2 beta (which had the first two levels of the Parish), we got to the trailer with the indestructible doors just as tank spawned. He tried to get in, but couldn't break down the doors. This was the beta, so we didn't realize he could break the skylight and come in. Sadly, he did know that and crashed down on top of us. We all ran back the way we came, but after exiting I quickly doubled back to the door. The last person was wounded and the tank gave him a smack right out the door, and as soon as he cleared it, I slammed the door on the tank's face. He couldn't break the door to get out and couldn't jump up to the skylight he had come in from. He was completely trapped. Most players know about this now, but I did it back in the beta, I seriously doubt I was the first person, but it wasn't the common knowledge it is now. The tank thrashed around in there so much that it eventually crashed the server and sent us all back into the lobby. I like to think I threw an immovable door in front of the unstoppable tank.
Just yesterday I had a pretty cool moment IRL too. I was playing airsoft with a friend shooting at him from behind a tree. He started flanking me but there was no cover nearby so I sprinted down the hill next to me to get to a small trench down below. I fired as I moved (crappy spring pistol so you had to pull the slide back after each shot) and ran down the hill. I took it a bit to fast and fell just as I reached the bottom. I landed on my stomach and heard my opponent jumping out behind me for a shot. I rolled over and cocked the slide on the pistol at the same time and fired at him without really looking, then scrambled up and ran for the trench I had been aiming for. I heard an "Oww... headshot..." and looked over my shoulder as I was running and saw my friend grabbing his head. We were playing to three hits and that was only the first. Because off that I ended up losing but it was still damned impressive.