Sources can be found at the credits section
The FGD file (a file that the level editor needs in order to know the entities of the game) of the leaked alpha version of Half-Life features some leftover entities from Prospero.
item_book Leftover from an unknown demo, as pointed out by the comments.
Since Prospero took place in a library, many theorize that it originally belonged to the game.
Self-explanatory. It damages the player when touched, and it can be disabled (for a short time). Likely a Prospero leftover.
trigger_libraryportal Similar entity to Half-Life's trigger_changelevel, except that it could change its targetname.
It's likely that this entity would make traveling between libraries work.
The Half-Life Alpha v0.52 mentions Prospero on an unused credits image, as a registered Valve trademark.
On the tech demo, this droid is used to showcase Half-Life's skeletal animation system.
It contains animations for attacking and dying, and given the nature of its design, it was likely meant for Prospero.
Some of the soundtrack files in the media folder are named "Prospero0X" from 1 to 5. Given the fact that the rest of the soundtrack carries the "Half-LifeXX" name, these were reused/repurposed from Prospero.
Vague Voices (Prospero01.mp3)
Steam in the Pipes (Prospero02.mp3)
Traveling Trough Limbo (Prospero03.mp3)
Dimensionless Deepness (Prospero04.mp3)
Threatening Short (Prospero05.mp3)