Hotornot.com + Google Maps - HotorNot.com is a site where you can post a photograph of yourself (or someone else) and have visitors to the hotornot.com website rank the photo. The question that is posed - Hot or Not? Think what you may, this site has been around a while and continues to be popular. This Google Maps mashup has taken the site a step further by plotting the locations of those in the photos on a Google Maps overlay. As a footnote, HotorNot actually has an API and developer wiki which this person used to create this mashup.
Edit a Google Map like a wiki - Here's a Google Maps mashup which has combined the 'Wiki' page publishing concept with Google Maps. "Maps can be created, edited, saved in a Wiki fashion". The creator is eager for feedback.
Google Maps and the TTC (Toronto Transit Commission) - I'm not sure if I already posted this overlay or not. If not, here is the Toronto subway network (known as the TTC) overlaid onto Google Maps. So much more could be done with the captions for this hack. Station information, accessibility info, photos etc. I'm sure more detail will be added to this or an enhanced mashup.