I did like that one puzzle in OpFor where you have to destroy some crates to let the loader bot continue it's work, which is setting down a (conveniently placed) crate that lets you walk over the toxic waste and flush the room clear. I thought that was cool.

As for normal old Half-Life, I can't think of puzzles, but I remember the scripted scene where a grunt throws a detpack into a giant pipe you're crawling through, forcing you to furiously backtrack to some water. First time that happened I died, but it was still awesome.

The first garg battle is kind of a puzzle, where you get it to walk in between those two electric tower beehive things, but I only did that on my third or so playthrough. Usually I threw grenades at it until it exploded, which is sort of how I thought you had to do it. Kind of like shooting down the apache with a handgun (which was also possible).

The biggest puzzle in the game was probably the tentacle thing, since you had to sneak through, bust open the boarded-up doorways, distract the tentacles with grenades, then activate the fuel and oxygen and power by using two completely seperate giant paths that had their own puzzles and deadly obstacles to avoid, and the catwalks were harder to get through if you destroyed them by blowing up the barrels that were on them. And then one of the paths involved flying up a giant wind tunnel (the least plausible part of the game).

Most of the "puzzles" are just optional ways to get through an area that doesn't involve shooting. Like there's the auto turret where the control lever is under a platform that's under the turret. You could of course shoot the turret a lot, which means taking damage. And then right after that is like 8 headcrabs, but you can take a nearby Barney along to have him use his ammo instead of you using yours. Once I lured the headcrabs into the path of the autoturret, for fun.

Some puzzles were just exploring. There was an office area with one locked room you couldn't enter except through the ceiling. To do that, you'd just have to explore. Also, if you explore some of the flooded parts in "On a Rail," there's a hidden area or two under the water that has some ammo and such. There was also the "bottom of a rocket" area where one lonely marine gets swamped with a ton of headcrabs sliding down the vents, that whole room is guarded by a bullsquid surrounded by a ton of bones and gibs.

Last memory for now: I thought the ending of Blue Shift is awesome. The part where you're ditched before enemies weld open the door and rush in shooting. A well placed detpack makes a nice surprise for them.