I proudly discovered most of these myself, a few in RE5 were shown to me by awesome online players. Thanks guys.

RE2
- there are no restrooms in the entire game, they were accidentally left out
- the kiss between Leon and Ada was meant to be an FMV (rendered CG like beginning and ending videos), it would have been the first Animated Kiss in Video game history.

RE4
- every now and then there are birds nests up in the trees, if you shoot them treasure falls out

RE5
- shoot the rats in the game and they'll drop gold
- you can sell all the treasures by pressing "V" in the PC version
- The Executioner is invisible invincible and impossible to kill until he knocks down the fence in the Public Assembly area.
- You can easily get the mind control device off of Jill by buying the cheapest machine gun, running up to her, and shooting the device off one bullet at a time. It saves a LOT of time instead of having to rip it off her chest little by little and risk her hitting you if you miss the quick time events. Also if you miss and hit her, it won't kill her, you can afford to hit her many times.
- when you're on your way to the helicopter wreck, you have to throw Sheva across a gap to another building, then cover her as she goes down and shoots the lock off the door freeing you. Don't. Just grab a grenade on the second story balcony, aim for a flat sign on the building across from the balcony and bounce the grenade off of it and it will land near the locked door below, and blow it off.
- You can take the lantern from the mines with you all the way to the elevator, but it goes out after your partner lifts one gate, and also disappears completely after you exit the mines.
- In the Ancient Caves you can actually shoot the enemies who are focusing the light reflectors on the floor.
- After Excella Mutates into the Uroboros monster, you run through the container ship and some areas are blocked off by her tentacles. If you shoot them enough they'll recede.
- I don't think the Chainsaw Majini can climb over windows (he couldn't in the demo, can someone try in the game?)
- Right after you and Sheva open the giant rolling gate to get to the Helicopter wreck, there are 3 red barrels you can shoot and explode, and one non-exploding one. They'll reappear as soon as the movie and quick time events start.
- You can shoot crows but I don't think they leave any treasure or gold. (confirmed?)

Those RE2 things were pretty nice. I thought that shooting the bird nests and getting treasures in RE4 was common knowledge? One thing I actually did quite accidentally in 4 was in the sewer area (or something like that, it's been a long time) and out of frustration started slashing the water thus getting a really big fish I was surprised to say the least.

In RE5 I actually ran a good time after one hell of a big rat in hopes it'd give you a lot more money than the others. Sadly, it didn't. I also quite instinctively shot the majinis using the light thingies in Ancient Caves. Some of those RE5 ones are something one could figure out if tried (I was actually going to try taking the lantern with me but figured it wasn't worth it) but still neat to know. Never tried the chainsaw majini thing but I always figured he could, maybe it was just some unfinished thing in the demo?

The last one is confirmed at least by me: shot all the crows, got nothing.

I knew about them leaving the bathrooms out in RE2. And yeah, the nest treasure is common knowledge. The executioner majini is not invisible ever but he is invincible until he knocks the gate down. Otherwise you could get stuck there. For Jill's device, I always used the M500 to shoot it off. One miss and she's gone though. I rarely miss. You can aim the gun through her and it greatly increases accuracy. Sniper rifles are the preferred method for it though...not a weak machine gun. Aim a sniper right in front of her and you pretty much can't miss. I got curious once and did the lantern thing as well. Actually...I did that on my very first playthrough of the game. I love sniping the light mirror majini right off the bat so you never have to worry about them while doing that part. I knew about the barrels. I was hoping they would stay gone in the intro but alas, they did not. And I also loved shooting crows out of the sky with my M500. I was actually getting really good at hitting them almost every time.

neat finds with RE2...knew that about RE4...havent played RE5 so I cant say a thing about that

@Superchris: I think you hold (x) down and change directions with
the left analog stick.

I use the big ass magnum on her because it's the most badass thing to do ever.

RE5- Chris's minigun backpack blocks arrows and most melee attacks from behind

Resident Evil 3

in RE3 if you try to use many time the realoding tools you can get new handgun and shotgun ammo.
in mercenary mode you can get more extra time by clicking on six specific locations. 2+4+8+16+32+64 = 126 sec