DOOM 1/2 + tons of maps since 1994. It is the best game ever made.
Q1 since 1996 right after tech demo QTest (Feb 1996) and later after shareware/full was released (June 1996). Tech demo was just a few maps with no sound. There was no real test/demo before full release, if i remember correctly.
Q2 since December 1997, i really liked this game, much more than Q1 single player and it also was the first game to utilize 3D acceleration. I bought 3dfx Voodoo 1 because of Q2 and it made such a difference it was almost unreal. At that time i lost my interest in Quake 1 almost completely.
Q3 since early March 1999, after IHV release was leaked out by ATi. It happened on weekend before planned Daikatana demo, so i wonder if this leak was not maybe intentional to divert public attention from Romero's new game to Q3. IHV was not really playable, but people did experiment with few servers and fixed maps with manually added items/weapons. Then Q3Test was released on April 24, 1999. It had 2 maps q3dm7 and q3dm17, and game was fully playable and big success even before Q3 was finished in December 1999.