I'm probably the only one who does this, but I still find myself playing Star Trek: Armada 2 when I'm looking for some boredom relief.

Dungeon Keeper 1+2
Heroes Of Might And Magic II
The Settlers II Gold
Diablo II Lord Of Destruction
Civilization 2
Interesting... Looking at that list now, I notice it's almost exclusively the second installments of various series. I'll make a general thinking out loud here, about how the second installment usually fixes the things they got wrong the first time, while later installments suffer from "the need to innovate", thus incorporating new features that don't really fit the game.
Still, I like Dungeon Keeper 1 better than 2.

Well, I play quite a lot of C64 games still. Wizball is my absolute favourite, but there are many others.
IK+
Giana Sisters
West Bank
Buggy Boy
Nebulus
And countless others...

Little big adventure 1 & 2 (to run the first one under windows just look for "lbawin", the latest version works flawlessly, game completed without any single bug)
Age of Empires 1 & 2 (I hate the third)
Warcraft 3 (the world editor is simply addictive)
Jedi Knight: Jedi Academy : the multiplayer can't get old for me
There are others but those came up quickly at least. And I certainly spent an impressive amount of time on them all.

I love the Origin's RPG, they are great, Underworld 1 and 2
-Tie Fighter (original version) I've created countless missions using Tie Builder!
-Total War: Rome (thanks to awomo.com)
-Wolfenstein: Enemy Territory, you would be surprised it has one of the most prolific multiplayer scenes i've saw (considering how old it is thats even better)
-Colonization
-Operation Flashpoint (so real! im thinking of buying it in GOG)
-Thief

Super Metroid and Castlevania Symphony of the Night.
Drives me crazy that all the games that are still 2d in those series keeps getting released on handhelds. I hate playing on a micro screen :)
So I go back and enjoy those classics.
Later when I'm getting a PS3 for Resident Evil 5 I'll also grab the 2D remake of the classic Bionic Commando :)

I am very-very hope that GOG will give us LBA1-2 sooner or later. Those two are definitely on my TOP10. :-)

I always find myself going back to the Monkey Island series, as well as The Betrayal at Krondor and Betrayal at Antara games. I really wish Betrayal at Antara would work! *hint hint to GOG team*

Yeah, I hope to see LBA 1 & 2 here eventually. I would buy them in an instant. I'm thinking at these games since I registered on GoG. I wonder who is holding the license now, because Adeline is dead.

One or two times a year, I install the Curse of Monkey Island and my computer and play through it. I practically have all the puzzles memorized (even on mega monkey mode), but the dialogue and characters are so well done that its always enjoyable. Grim Fandango is another beloved adventure game, but I don't think the gameplay has aged as well as Curse.
Also, last year I went back and played through Baldur's Gate 2 again. It absolutely blew my mind. I had played through it (using a strategy guide) about 5 or 6 years ago, but I decided to go through it casually without any help. The game is so well designed with so many great quests. It was like a completely different experience, and one that made me look back and realize how crappy KotOR (and their other newer games) are in comparison.

Heroes of Might and Magic III - I install it, I play, I format my HDD, I lose my Heroes saves, I reinstall, etc...
C&C Tiberian Sun - See Heroes III
C&C Red Alert II - See Heroes III
RTC Wolfenstein - An good FPS with weapons variety, Nazis for targets and (somewhat) a scenario

Blake Stone
I grew up playing that game. I love it. And since 1995 I reinstall twice every year or so to beat episode one again and again (i grew up playing a shareware... yea I was young. But my whole family played it)
Old Command and Conquer series
From Tiberian Sun to Red Alert and the original Command and Conquer, I always go back to them... great games

Civilization IV.... over and over and over...
Baldur's Gate 1 + 2 ---> each time a different experience.

Jagged Alliance 2, UFO: Enemy Unknown - two great tactical games I keep going back to. Funny thing, I tried UFO after JA2 (late 90s) and still liked it very much.
Championship/Football Manager - this is the game I've spent most time playing.
Hearts of Iron 2 + mods - it will probably be still like for a couple of years after HoI3 release, because it always takes some time until Paradox's games are properly patched and balanced.

Quake - Even though it has zero plot the clash between gothic techno always lures me back. Not to mention deathmatch.
Doom - I haven't installed it in so long but I figured I should mention it since I use to be an addict.
Alpha Centauri - It always draws me back. It's the first game besides Civ that I've found let's you plan out everything and be creative. That's one game I thought would be crap and was really surprised when it wasn't.
Red Alert 2 - Awesome story and a great game. All around perfect balance of everything.
Empires Dawn of the Modern World - This is a hands down great game. Much like any other RTS game out there but the war machine is balanced perfectly. Modern weapons versus ancient ways. All I've ever played is skirmish mode but I'm sure single player is just as fun.
Daggerfall - I still have to finish it.
All the Sierra Adventures - What can I say they made some damn good adventure games. If you haven't played any of them or don't know how, go to youtube. Either watch a let's play or find out how to play them.
All the Lucas Arts Adventures - Same rule applies from Sierra except the visits to Lucas are like using a public restroom. You have to go really bad but you really don't want to touch anything. If you do you'll be standing there for hours wondering why the sink doesn't work.
TerraNova - I've never found a plot about space pirates more interesting. It's amazing that there's not one single alien in the game and it still holds together. Hooking into your mech suit and blowing the crap outta stuff is always a good time.
Fallout 3 - Playing both PC and 360 versions. Good on one evil on the other.
Carmageddon 2 - Racing around a track and watching peds explode as you hit them at full speed while listening to heavy metal. A match made in Heaven.
Panasonic 3DO - They made some really good overlooked games for it. When it came out I was to poor to own one. Now I'm just poor enough to own one. My 360 has taken a backseat for some time because of it.
One thing I've noticed over the time I've spent on this planet is that games aren't like they use to be. You bought one game and that was it for 3 to 6 months. Just because games were that good and involving. Once in a while you find a pile of crap but those companies went out of business quick. Now I find myself buying 1 or 2 games a week just to have something fun to do on my days off. But it's not just me, my buddies do the same thing. So I'm thinking that games have taken a slide into the turd dumper but there's so many games we haven't noticed what's happend. So my guess content wise there's nothing in the games to keep you playing.
That's why I've turned to buying classic and vintage PC games. I've found that I'm alot more content with my purchases. You get a much better feeling then downloading them off some abandonware site because it's more then just another throw away game then. That's why I'm glad I found GOG not only can I buy PC games but hang out with people who enjoy them to.