Me, being a hardcore perfectionist, would like to spin as many Pokestops as possible. How am I supposed to know what stops I haven't gone to before? Is there some kind of journal that I am unfamiliar with? Keep in mind that I already know that whenever you spin a certain place, the Pokestop turns from blue to purple/pink.
Please see the answers below, as they note a feature that I was not aware of previously, and they are better responses than mine :)
My (incorrect/outdated/whatever) answer, left for reference.
There is a history log of everything you have done so far in terms of pokemon encounters, gym fights and Pokestops.
Select your trainer image in the bottom left of the main screen, then click on the "Journal" button just below your XP bar.
While this does not tell you the location of the pokestop, it does tell you when you went there and what you received.
Considering Niantic's previous game (Ingress), and how it tracked "unique portals" (their equivalent of pokestops/gyms), I would imagine the data is stored somewhere in their servers, but it is not made public. However, it could become public at some point in the future, and I would probably expect it to be connected to the journal (or just become a new feature).
The game, as of version 0.29.0, offers no public way of seeing where you have gone before. If you would like to track this information, consider using a map, and just mark off the places you've visited.
Additionally, it gives players no incentive to actually visit unique Pokestops as they (as of now) do not count towards any badges or drop any rewards.