Got you covered.
Grab freedoom or doom iwads, and you can play the ever increasing caboodle of fantastic doom (mega)wads. Sunder, Sunlust, Valient, BTSX, Eviternity – there are genuinely too many to list.
For campaigns I can recommend Ashes (2063/Afterglow), Adventures of Square, and Rekkr.
Do the same with the Red Alert 2 Yuri's Revenge files and you can try Mental Omega. That's a good 200 hours right there, not even counting skirmish.
OpenMW with Morrowind, which gives you access to Tamriel Rebuilt and Project Tamriel.
Blue Wish Revolution+ is a great, free STD that can run on windows 2000. There are more out there, but I'm not that well versed in doujin shmups: I'll leave those recommendations to our resident STGhead.
Also try flashpoint. Tons of fun timewasters on there if you're willing to look.
That goes for all of video games by the way. Many of the best games ever made were released before the PS3/Xbox360 were even conceptualized. Only requirement is willingness to put in the time.
Finally, the best version of Spelunky is free and resource light.
>>32806Good list, though Dwarf Fortress or Cataclysm will be sloooow. Starsector is also a great simulation game that you can sink hundreds of hours into.