The Quest for Melchiah
The Quest for Melchiah is a cut-down version of Soul Reaver distributed by The Learning Company. While the packaging does not describe it as a demo, that's what it is nonetheless, comprised of the same events as the full version of the game up until the battle with Melchiah.
Interestingly, the Human Citadel is also included in its entirety, even though the Water Glyph area is inaccessible without the swim ability - which cannot be gained in Quest for Melchiah (without using cheat codes or hacking).
On the other hand, the Stone Glyph area is also missing and it is possible (in the Spectral Realm) to reach an "end of the world" type of point where the tunnel to it should continue. Don't jump off the ledge unless you want to restart your game.
Quest for Melchiah is built around a full version of the PC Soul Reaver engine. It is possible through the use of active memory editing to enable every ability that is present in the full version. However, the engine version appears to be an early one (possibly 1.0?) because the audio (at least on Windows XP) has serious issues and makes it next to non-playable.
Another item of note is that the Quest for Melchiah CD contains all of the FMVs (intro, chronoplast, verse, credits) even though none of them (even the introduction) are used in this version.
Quest for Melchiah contains no material that is not present in the full version of Soul Reaver. Its purchase should be limited to hardcore fans, as the full version of the game can generally be obtained for about the same price.