This is pretty much exactly what the title says it is; a list of 100 system agnostic ideas of signs that the restless dead are present. Between this and 100 Ways to Become a Ghost, I think the latter is slightly better in execution as more of the list seems usable to me. Some of the entries in this product are either too vague to be helpful (such as #97 Warning), or seem like they be more appropriate as a the basis for a supernatural encounter rather than as the sign of ghosts being nearby (such as #27 Flying banshee).
Still, enough of the entries are usable to make this more than worth it's $1 price tag.
[3 of 5 Stars!]