Frontier: Elite II
This instalment in the Elite series enjoys quite a huge popularity. The game itself had been released first as shareware and then as freeware, according to Wikipedia and Elite Wiki. I have not yet found the original shareware package (if it were ever released online that is), but the freeware release (which is shareware with included activation codes) can be found at numerous places, including FilePlanet, FrontierAstro and Frontierverse. There's also an early non-playable DOS demo: FRONTIER.ZIP
Frontier: First Encounters
A sequel to Elite II, which had also been released as shareware. The details about the freeware release are rather sketchy (with the archive.org's servers that host the old pages of eliteclub.co.uk and frontier.co.uk down not helping matters), but the shareware version can be found here. Note that both shareware versions of Elite II and First Encounters include the following clause in the license file:
Star Crusader5. DISTRIBUTION.
Limited Distribution License for Redistribution of Evaluation Copy.
You may without making any payment to AUTHOR make copies of the evaluation SOFTWARE as
you wish; give exact copies of the original evaluation SOFTWARE to anyone; and distribute the
evaluation SOFTWARE in its unmodified form via electronic means (either directly or through the
posting of a copy of the SOFTWARE on the Internet, BBS's, Shareware distribution libraries, CD-
ROMs, etc.) to third parties who are interested in evaluating the SOFTWARE except for the
purpose of extending their evaluation period
Like Frontier games above, this game had been published by GameTek. The game is not free but a playable demo version is available: STARCRDE.ZIP
Wing Commander III: Heart of the Tiger
A playable demo is available either as a single file or split into smaller parts.
Single archive: wc3demo.zip
Split version: wc3demo (folder)
Wing Commander IV: The Price of Freedom
This game has two versions of the playable demo, one of which is smaller in size.
Smaller online demo: wc4demo.zip
Large demo: wc4demofull.zip
Wing Commander: Prophecy
Playable demo: demowc.exe
Demo movies: movies.exe
Wing Commander: Secret Ops
A stand-alone add-on to Prophecy, this game has been released as freeware over the Internet, with additional episodes released as separate downloads.
Main installer: secretopsfull.exe
Episode 2: episode2.exe
Episode 3: episode3.exe
Episode 4: episode4.exe
Episode 5: episode5.exe
Episode 6: episode6.exe
Episode 7: episode7.exe