This free iPhone application allows you to tag locations on a Google Map. The application lets you search for interesting places nearby, view other users' MapHooks, read Wikipedia entries about local points of interest or read Yelp reviews of nearby businesses.
When you leave your own MapHook at a location you can attach photographs, submit information and add a category to the MapHook. You can also attach Wikipedia entries and Yelp reviews.
It is possible to set different privacy settings for each MapHook that you post, giving access to 'Everyone', 'Co-Workers', 'Family', 'Friends' or 'Me'. MapHook also allows you to combine your entries so that you can create a collection of MapHooks that could, for example, record places you visited on a vacation.