Recomend me some awesome games on GOG

You can't really go wrong with GOG.com. Pretty much any game they have is going to be good if it doesn't have only 3 stars (e.g. Master of Orion 3). Aside from those three or four games that are "to be avoided," every title is a good one. Just read the reviews to know what a game is like, spend the few dollars, download it, and enjoy. If it somehow turns out that the game doesn't work for you, they have a money-back guarantee.

• Blood
• Redneck Rampage

The Might and Magic RPG's ( 6-8 are probably the best to start with ). Theme Park was also recently released on GOG.
Thing is: more than 80% catalogue from GOG deserves to be owned and played. Games might be old, might not be shiny, but definitely are giving a better satisfaction and lots of fun.

• Shooter: SHOGO: MAD
• Adventure: Sanitarium
• Platformer: Pandemonium 2
• Racer: Megarace 1+2 (for the second game only, really)
• Like Really Fucked Up "Racer": Carmageddon games
• RPG: Neverwinter Nights
• Management: Theme Hospital
• Simulation: Freespace
• RTS: Earth 2150
• Emotions: Omikron: The Nomad Soul
• What The Fuck Am I Even Playing Edition: The Incredible Machine Pack (I'd pick this if you're not sure what genre you want to play)
• Halo Is Now Third Person And It's Kinda Crap: Advent Rising (shut up, I liked it)
• Nightmare Fuel Where You'd Least Expect: Thief Gold
• Not On GOG And Why: Ecstatica

Might have gotten a bit carried away there. Anyway, whatever you pick from there, shall make you happy.

Blade of Darkness

I would recommend Arcanum if you dont feel like playing Fallout 2 one more time. Also Planescape: Torment is really good

The first thing I thought to recommend was RollerCoaster Tycoon, but I see you already have that. XD
So in that case I will recommend Riven. I never really got anywhere in Myst, but its sequel has been one of my favorite games since I was a kid. A really good puzzle/adventure game.
Also, not sure how well it would run on an older computer, but another one of my all-time favorites is Psychonauts. Hilariously quirky adventure/platformer.

total annihilation

If you like shooters, you should go for Shogo. If you want RPGs, go for Fallout 1 and 2, or Planescape: Torment.
Temple of elemental Evil is a great game as well, but it's so difficult it's frustrating.
If you like Civilization you could check Master of Orion out. It's kind of like Civilization, but with aliens, and colonizing galaxies.

I still want to play through Arcanum again, but keep playing other things first. Last I knew, Arcanum was available from GOG.

Everyone else in this thread has already suggested everything I would, but if you need more, there's a GOGmix for games that run on netbooks.

Also, Rise of the Triads.

Wing Commander I & II. With the packed in DosBox, it works perfectly on modern computers and modern controllers. Plus, now it comes with the expansions.

I Have No Mouth And I Must Scream is one of the coolest games ever made, definitely check it out if you are at all into horror games.