Demo

A demo or shareware version of various games in the Diablo series has generally been available to let people try out the game in part without buying it.

First game
A disc for Diablo (game) was included with those who bought a copy of the game, allowing them to give a copy to friends who wanted to try it out.

Diablo Spawn was one name that referred to this.

In addition to being on a CD (which was legal to copy) this was also legally downloadable from the Blizzard website.

It only allowed playing the Warrior (not the Rogue or Sorcerer) and only the first three levels.

There was no equivalent of this for the expansion.

Second game
Diablo II had two free versions available with different formats.

The closed beta allowed all five classes playing skills up to 18 in the entire act 1.

The open beta stress test allowed only Barbarian playing 1st tier skills up to Blood Raven.
 * the shareware version was the same as this, and both had dedicated servers which could not interact with full-version characters.

Third game
A Starter Edition accompanied the debut release of Diablo III. It was initially only available to people who has a guest pass code from someone who bought it, but months later became available to all.

It limited Act 1 exploration and quests up to the Skeleton King. There was initially a level cap of 13 but this was later changed to 20.