One feature I find myself missing in today's games is .. secrets.
Yes, secrets. Like when you accidentally trod on that tile and had a door open behind you with some goodies. Or when you shot that slightly irregular wall on a hunch and find it opens to yield the medkit you so sorely needed.
Remember the Wolf3D map with 99 secrets? How Duke3D would always have secrets missed about three times larger than secrets discovered on the summary screens? How Blood had its first secret about five yards from the coffin you started in and just kept on giving? How you would always explore the interiors of a secret very carefully, should it contain another secret (and another, and another)?
These days secrets are nowhere to be found in games. Doom 3 was horrible, all it had was little closets strewn about containing ammo you probably didn't need. The only half decent secret in Doom 3 was the ID PDA at the very end, but this was disappointing as well since it was so damn easy to find. I mean, I stumbled into that wall on my first ever playthrough and it wasn't all that hard to connect the dots.
Most other games of today don't have them at all. It seems that adding secrets to todays games is just an unecessary development bump, which has been optimized away with. Why spend development dollars on implementing features so few of the player base will ever see?
Anyway, it's something I miss, especially in FPS's. Being rewarded for exploring every nook and cranny of a level should be a good thing, yet it is rarely seen in newer releases.
This thread is dedicated to secrets in games. Post about your favourite secrets. Name the games who have them.

I miss them too, secret levels, secret weapons, yes..

So, anyone know anything about the cow level? I heard the witch outside Tristam is hiding it in her cellar, we should all walk there. Yeah.

Hehe, the secret cow level is awesome! Especially the horribly tacky voice acting, which is clearly some of the developers going "Moo!" into their headset mics. I played it again only a month ago :-D

it was very, strange that after i've killed all enemies on map i was often looking for this one more secret i missed sometimes for over half hour. empty map, just walking and hitting use/open button or shooting to walls. Im still doing it plaing duke ;] and it's never get boring ;]
I remember finding this dopefish graffiti in quake I, made a screenshot and print it to show friends in school , really cool. I think most fps had them them.
From more recent games i remember that in first FEAR was room with radio plaing tune from SHOGO. And there are secret places in Necrovision at least in demo ;]
I want secrets back

Serious Sam (both TFE and TSE) has a load of fun secrets.
Each secret is an easter egg really.

Well, some freeroam games still have them, but those aren't really The secrets (though attack fork from Morrowind was awesome :D).
And yeah, I do miss them ... It was great to stumble upon some secret stash, it was great to have rocket launcher in firs level of Duke Nukem 3D (heh, there are even 2 of them around :D )

Well, not all secrets are about stashes. Some are just plain easter eggs. Like this one, from Alien Shooter: Vengeance. It's the developers' office, with pictures of them on the walls. I've seen similar secrets in other games.

I would hold that some modern games still have secrets, just different kids.
Half-Life has made it a running thing to give you small glimpses of the G-man.

Seeing the G-man is a narrative aspect of the HL series... it's not quite on the same level as finding a sam and max reference in other lucasarts games...

Here's one from Day Of The Tentacle.
I cut that shamelessly from the map of the Edison mansion from the past on this site, which has maps of lots of old video games. Incidentally, it also has a GOG.com banner on every page :-D

Yeah the secret cow level was a good nod to the fans after the big D1 "secret cow level" that never existed. Plus it was a great place to level and MF until LoD came out. Kudos to Blizz on that one. And yeah, the monotone "Moo." and the urgent "MOOO!" from the developers was great. Hey D2 was just great, and still has a ton of people playing on the closed Battle.net realms despite a game that was released in 2000 with 1998 graphics.

Yeah secrets and to some extent easter-eggs were cool.
They've bleed into the whole Achievement thing now, and while achievements are good... It's not in the same field as stumbling across a dopefish swimming in fish tank aboard a supervillain 's ship.

It's because of the realistic setting of most games; pushwalls and the like would be an immersion killer.
That said, I also think a lot of it is just neglect on the developer's behalf.

I think 2 of my favourite easter eggs/secrets are the Foster encounter in the london underground and the puzzle bypass towards the end of Broken Sword 2 (I've always said that boobies can open doors)