Ooo, you should try these two, as I regard them as classics (although I might be biased due to how they were two of my childhood games):

• Empire Earth/Empire Earth I (i.e. don't try Empire Earth II or III) (an RTS); and,
  
• Age of Mythology (an RTS).

EDIT: I forgot to mention both are pretty aged, however Empire Earth (I) more so. Nevertheless, I still find enjoyment in them. Oh, I should also mention that you might have difficulty installing them on Vista (pretty sure its fine for 7, and definitely for XP), but using the Compatibility settings set for XP works.

Mostly the above posters got it down, but just to add a bit of a off-topic game: http://en.wikipedia.org/wiki/RollerCoaster_Tycoon.

It is old school, but realy, really addicting. I would still think some places still sell it in a "Deluxe" edition or whatever, so keep an eye out for it (Should be like 10 bucks).

http://i.imgur.com/sUMbq.jpg

and don't forget multiplayer, where truly believable worlds are crafted.

Thanks everyone. I have played a wide majority of the games you all have mentioned but I guess I forgot.

Warhammer II 40K, a great mix of strategy and rpg like stats and equipment.

Ever play the FATE games? They're more or less dolled-up Diablo clones, but they're still pretty fun to play, and all moddable, too! You might like them if you like Diablo, Torchlight or Dungeon Siege.

I can't really think of any others to recommend that you might get into, unless you like The Sims...

Tim (aka the Slipperman)

Dawn of War 1 is as much a jewel as it's successor, very different game play but it's incredible good as well..

Age of Empire
Rome Total War
Zoo Tycoon
Rollercoaster Tycoon
Minesweeper

Pathologic deserves playing. Prepare to love and hate it though.

turn based RPGs
-King's Bounty : the Legend
-King's Bounty : armored Princess
-Heroes of might and magic 6 .. but it was a bit boring , king's bounty was way better .

RTS
-The Settlers 7 was great game if you don't mind Ubisoft DRM
-Anno 2070 , main story missions is a bit short , but the scenario will keep busy
-Stronghold 3
-and you can always play Warcraft 3 custom maps on battle.net , TD maps .. DotA .. RPG maps

Action hack n slash
-Darksiders the game released back in 2010 but it's alot of fun

I'm not much of a RTS guy but my favorite RTS is Command & Conquer: Tiberian Sun. Followed by Warcraft II, Generals and Starcraft.

Tiberian Sun must be free these days.

X-Com and Syndicate (the originals, of course ....) weren't mentioned yet?

You can pick up the whole C&C collection for super cheap, I think it was $10(USD) when I picked it up. Tiberian Sun is really the only one worth playing. Speaking of free PC games http://www.rockstargames.com/classics/ thanks to Rockstar! Of course you can get http://www.elderscrolls.com/arena and http://www.elderscrolls.com/daggerfall/, but I bet they've already been linked. There are plenty of newer games that were commercial, too, some are even great. http://news.bigdownload.com/2008/10/07/download-full-spectrum-warrior-full-free-game/ http://www.fileplanet.com/187537/180000/fileinfo/Psi-Ops:-The-Mindgate-Conspiracy-%28Free-Game%29

http://en.wikipedia.org/wiki/List_of_commercial_video_games_released_as_freeware

I don't know if anybody mentioned it already, but Sins of a Solar Empire is awesome. It's a space RTS that blew me away the first time I played it. Torchlight is a pretty good dungeon crawler, you can grab it for $5 on the right day! Check http://www.gamesas.com/topic/1323066-game-sales-and-deals-steam-gog-impulse-desura-etc/ for (kind of) up-to-date deals that are happening on some of the major websites.

I don't know if Torchlight is worth it at this point, Torchlight 2 should be right around the corner. Unless Torchlight 2 has some major unforeseen flaws it'll probably make the first one obsolete.

Oh, and http://store.steampowered.com/app/24400/?snr=1_7_suggest__13