Here is Steam's news release; every title id released to date is available for download.

I must admit that I already own copies of most of them (see full list below) but you can buy the id super pack (22 titles/addons) for $63USD. I think it's worth that just to have everything available for download via Steam rather than having to remember where the original media is.

Quote:
id GAMES on Steam
• Commander Keen, Episodes 1 through 5
• Wolfenstein 3D
• Spear of Destiny
• Return to Castle Wolfenstein
• Ultimate DOOM
• DOOM II
• Final DOOM
• DOOM3
• DOOM3: Resurrection of Evil
• QUAKE
• QUAKE Mission Pack 1: Scourge of Armagon
• QUAKE Mission Pack 2: Dissolution of Eternity
• QUAKE II
• QUAKE II Mission Pack: The Reckoning
• QUAKE II Mission Pack: Ground Zero
• QUAKE III Arena
• QUAKE III: Team Arena
• Heretic
• Heretic: Shadow of the Serpent Riders
• HeXen
• HeXen: Deathkings of the Dark Citadel
• HeXen II

id PACKAGES on Steam
• DOOM 3 Pack: Includes DOOM 3 and DOOM 3: Resurrection of Evil
• QUAKE III Pack: Includes QUAKE III Arena and QUAKE III: Team Arena
• Wolf Pack: Includes Wolf 3D, Spear of Destiny and Return to Castle Wolfenstein
• DOOM Pack Complete: Includes all DOOM games released by id
• QUAKE Pack: Includes all 8 QUAKE Titles available on Steam
• Heretic\HeXen Pack: Includes Heretic, Heretic: Shadow of the Serpent Riders, HeXen, HeXen: Deathkings of the Dark Citadel, HeXen II
• Id Super Pack: Every id game on Steam

Select packages are 10% off until August 10.