Google Shared Spaces
Google Shared Spaces is a Google Labs project that allows you to easily create a shareable space with a collaborative gadget and a chat box. There are already a number of collaborative gadgets that you can use in Shared Spaces, including games like Soduku and a number of Google Maps based gadgets.
TravelWithMe allows groups of people to plan trips together in real time. Once you open TravelWithMe you can start planning a trip by simply grabbing locations and dropping them in your itinerary.
The map includes a calendar that allows you to add a date to each destination. If you share your TravelWithMe map with others you can plan a trip collaboratively.
Map Gadget lets you collaborate on a map of placemarks, paths, and shapes with other participants. Using the map you can draw shapes on Google Maps or you can just share locations or draw paths.
Map Cluster Gadget
The Map Cluster Gadget lets you create a Google Map to share locations. Everyone who opens the gadget can add their location to the map and also see where everyone else is from. This map is a great way for a website to create a map of readers' locations.