However, the Wayback Machine has, at least, preserved the demo download page listing all available files and their approximate size (regrettably, not the exact size in bytes, which complicates matters a bit). As Interplay was a large and well-known developer, many files are not hard to find, but there isn't one single place where all may be obtained.
The demo download page, in its iterations from various time periods, displays most files - at least, from the DOS era - as plain ZIP archives. However, some of the distributions had self-extracting PKZIP counterparts, which are a bit less preferable but still original alternatives if the ZIP files are nowhere to be found.
Sources that appear most reliable include FilePlanet and the SimTel, the Coast to Coast Software Repository mirror. Some file are of course found elsewhere, including some archives preserved by the Wayback Machine.
On a side note, Interplay also released some demo versions that were, for whichever reason, are not mentioned on the downloads page (and, presumably, were not available from the FTP). This includes the demo of Battle Chess II (BC2DEM.ZIP), the Blood & Magic demo (bamdemo.zip) and possibly some others.
Here's the list of the playable demo distributions, with links to working downloads if known/available:
- Alone in the Dark II Demo: A2DEMO.ZIP
- Alone in the Dark Demo: ALONE.ZIP
- Apache Havoc Demo: APHAVOC-11.EXE
- Atomic Bomberman Alpha Demo (Version 2) w/add on sounds: BM95ADEM.ZIP
- Atomic Bomberman Alpha Demo (Version 2) sound add-on: BM95SND.ZIP
- Battle Chess 4000 Demo: BC4DEMO.ZIP
- Battle Chess Demo: BCDEMO.ZIP
- Blackthorne - 3 Level Playable Demo: BTDEMO.ZIP
- Buzz Aldrin's Race Into Space Demo: BUZZDEMO.ZIP
- Carmageddon Demo: CARMDEMO.ZIP
- Carmageddon 2 - Carpocalypse Now!: CARMA2DEMO.EXE
- Castles Demo: CASTDEMO.ZIP
- Conquest of the New World Demo: CNWDEMOA.ZIP
- Conquest of the New World Demo w/Enhanced Sound: CNWDEMOB.ZIP
- Conquest of the New World Demo w/Enhanced sound and movies: CNWDEMOC.ZIP
- Descent 1.4 Preview version: DESC14SW.ZIP
- Descent: FreeSpace - The Great War Playable Demo: FSDEMO10.EXE
- Descent II 3-Level Interactive Preview: D2DEMO10.ZIP
- Descent III Interactive Preview version 1.1.1: D3DMO111.EXE
- See the Descent III Site for important Compatibility information!: DESCENT3DEMO2BETA.EXE
- Die By The Sword Demo: DBTSDEMO.ZIP
- Die By The Sword Demo - Voice Pack: DBTSVOICE.ZIP
- Dungeon Master II Demo: DM2DEMO1.ZIP (dm2demo.1exe)
- Earth 2140 Demo: EARTH.ZIP
- Evolution Demo: EVOLDEMO.EXE
- Fallout Demo: FALLDEMO.ZIP
- Fragile Allegiance Demo (With Sound): FADEMO2.ZIP
- Fragile Allegiance Demo (W/O Sound): FADEMO3.ZIP
- Freespace 2 Interactive Demo: FS2DEMO10.EXE
- Jet Fighter III beta demo: JF3TEST.EXE
- Lexi-Cross Demo - The full game is no longer available: LEXIDEMO.EXE (lexidemo.zip)
- The Lost Vikings Demo: LOSTDEMO.ZIP (lostdemo.exe)
- Mario Teaches Typing Demo: MARIODEM.EXE (mario.zip)
- M.A.X.: MAXDEMO1.ZIP
- M.A.X. 2 Demo: max2demo.exe
- Revised MDK Demo: MDKDEMO.ZIP
- Norse By Norsewest - The Return of the Lost Vikings: NRSEDOS.ZIP
- Norse By Norsewest - The Return of the Lost Vikings: NRSE95D.ZIP
- Omar Sharif On Bridge Demo: OMARDEMO.ZIP
- Out Of This World Demo: OOTWDEMO.EXE (ootwdemo.zip)
- VR Sports Powerboat Racing Playable Demo: PBOAT.ZIP
- Peter Jacobsen's Golden Tee Golf: GTGDEMO.EXE
- Pro Pinball Demo: PPINDEMO.ZIP
- Rags To Riches Demo: RAGSDEMO.EXE (ragsdemo.zip)
- Redneck Rampage Demo: REDNECKD.ZIP
- Sand Warriors Interactive Demo: SWDEMO.ZIP
- StoneKeep Non-Interactive Demo: SKNIDEMO.ZIP
- Solitaire For Windows Demo: SOLWDEMO.ZIP
- Star Reach Demo: SR_DEMO.ZIP
- Shattered Steel: SSDEMO11.ZIP
- Starfleet Command self extracting demo: SFCWWWDEMO.EXE
- Starfleet Command zipped Demo: SFCCDDEMO.ZIP
- Star Trek: 25th Anniversary Demo: TREKDEMO.EXE (trekdemo.zip)
- Star Trek: Judgment Rites Demo [1 of 2]: RITESDM1.EXE
- Star Trek: Judgment Rites Demo [2 of 2]: RITESDM2.EXE
- SWIV Demo for Windows 95 (requires DirectX): SWIVDEMO.ZIP
- SWIV Demo for Windows 95 with 3DFX video cards (requires DirectX): SWIV3DFX.ZIP
- Virtual Pool DOS Demo: VRPOOL10.ZIP
- Virtual Pool Windows 95 Demo: VP95DEMO.ZIP
- Virtual Snooker Demo: VSNOOKER.ZIP
- Virtual Tennis Demo: VRTNSID.ZIP
- Whiplash Demo: WHIPDEMO.ZIP