Googles Keyword Planner Tool provides useful data about how keywords perform in search for AdWords & PPC, and can be used to find keywords related to certain queries or websites.

SEOs can find it useful as a barometer that shows how they might anticipate a certain keyword would fare in terms of demand & what how competitive it is.

For SEMs additional information is provided that gives the average bid cost for the query, as well as where certain bids would land in terms of the SERPs fold.