These are two of the best FPS games ever.
Serious Sam often gets lumped in with other old school shooters such as DOOM and Quake, but I think it's much more unique in how it plays, as mentioned in the video above. While DOOM and friends put you in smaller spaces and the occasional open area with a few enemies, Serious Sam mostly keeps you in extremely large open areas and throws literally hundreds of enemies at you, and I believe some levels have over 1000. The fights are intense, as you are forced to swap between a large arsenal of different weapons for the right enemy and situation with little time to think. It is a nonstop barrage of action, action ACTION! You'll be using a flamethrower to fight off the skeleton horses and quickly swapping to your sniper to one shot the Scorpion w/ a chaingun arm in the distance right after you lob five grenades at the nearby Biomech. Can't stop moving, can't stop shooting. There are some powerups to help make the combat even cuhrazyer in the form of a damage and speed boost. With the damage boost enemies will explode in milliseconds after you shoot them whilst you're zooming past their projecticles.
Someone recently said that Serious Sam sucks because there's no level design or gunplay, but I disagree. While there are plenty of flat, open arenas, and Second Encounter has a lot of "go to the west area then the east area then come back" there are also many large areas with tons of buildings, levels of terrain and other objects to make the battlefield both hectic and strategic. You'll have to listen for certain enemies coming and plan ahead, or you can hide in a building and take everyone out as they flood in. You can jump on the charging bulls and ride across the map on top of them for a short time, raining death on everyone around. There was one level where I rushed ahead of a huge herd of skeletons and bulls, and lured them into a gated area where they weren't supposed to be. In that arena, tons of Kamikazes(enemies that rush and suicide bomb you) started spawning. Now, that would have been a fairly easy challenge, but with the group of enemies I lured in I had a hell of a time finishing it. I had to jump on one of the bulls and use a minigun to destroy everyone while constantly spinning around to shoot the Kamikazes before they got close enough to hurt me and dodging the skeletons who charge and leap you while taking care of the other bulls. It was one of my favorite moments in any video game ever. To have something unscripted like that create something so batshit insane and seemingly impossible but to overcome it was incredible. The indoor / smaller areas have multiple paths and are filled to the brim with secrets and easter eggs. What makes these levels fascinating to me are the multitude of optional, hidden battles. You may wander off into a sideroom, pick up an ammo pack and summon a horde of Kamikazes. There are plenty of these battles hidden throughout and it's extremely fun to hunt them down. On top of this, there are also lots of "trap" rooms, mainly in Second Encounter that mix up the gameplay. There is one room where the floor is essentially a trampoline, so you and every enemy bounces around nonstop. This is fucking hilarious with the Kamikazes running at you in midair along with rockets and such flying everywhere. The guns are all varied for the most part, with the major exception being the laser gun and the chaingun which feel very similar. But you have a nice arsenal that handles fairly different and as I mentioned above, you will have to switch on the fly for the correct situation. I think the best weapon in the game is the Cannon. It's literally just a huge cannon that shoots chargable cannonballs... However, the cannonballs rip through enemies like they're butter. It is orgasmic seeing a cannonball take out a horde of bulls, smashing and demolishing all of them one by one as their guts fly around. Or watching the skeletons crumble as you mow down tens of them with one cannonball. You can one shot the toughest of enemies with this gun. Of course, ammo is fairly limited to make it fair, but it is a blast using it. There's pretty much no melee but you do get a chainsaw in Second Encounter. Fun to mess around with, it's just not very practical.
As for environments, First Encounter is pretty much Egypt throughout the whole game. There's a whole lot of desert and tombs with a bit of grass once in a while. Second Encounter has a nice change. You'll fight through jungles, snowy towns, lava caves, castles, medieval cities and more. The HD versions look very nice too.
Croteam hired the band Undercode to do the kickass music. The metal instrumentals in this game are fantastic and really get the adrenaline pumping, especially during the boss fights. I mean, just look at this shit. You've got a bunch of enemies on your ass while trying to kill a God in the middle of an arena while you're going fast as fuck thanks to a powerup. Extreme.
Humor is a big focus in Serious Sam. Lots of funny easter eggs, dialogue and the silly looking headless enemies. You know those Kamikazes I mentioned before? Here's the noise they make as they run towards you. Imagine hearing that faintly in the background... You turn around. It grows louder. Suddenly, hundreds of unified AAAAAAAAAAAAAAAAAAAAHHHHHHHHHHHSSSSS fill your ears as a stampede of headless dudes holding bombs rush at you until you kill them or they kill you. Sam takes inspiration from Duke Nukem and is equipped with a pocket full of cheesy one liners. The story is presented mostly through text screens by your AI companion Netrisca(who later becomes a parody of Cortana) and is mostly skippable, but there is some funny dialogue once in a while. The story is focused on Sam being sent back in time to stop a Demon Alien Time Travellin' Overlord named Mental, whom he likes to taunt and talk shit to over the phone and such.
I'm head over heels with these two games. I wanted to explore the levels, every nook and cranny. The combat is extremely fun and I can't think of many games with something similar. I don't know what else to say. I encourage anyone who's interested to look into the Serious Sam Collection coming out soon on 360 if you don't play PC. I'd imagine this game is much more suited on KB+M but someone in that thread said they played through it just fine on a gamepad, so whatever. At any rate, while I hear Serious Sam 2 is kinda eh and Serious Sam 3 is awesome but not as awesome, I look forward to playing them. I of course am also very excited now for Serious Sam 4 and hope it sells a trillion copies.
P.S. Serious Sam Double D is fantastic as well. It's a 2D sidescroller / shooter where you can literally stack guns on top of each other. Fun game.