The launch of OpenStreetMap in 2004 paved the way for crowd-sourced map making. Google itself launched its own Map Maker tool in 2008 to allow users to add details and make amendments to Google Maps. Because of licensing issues with the some of Google Maps' data providers however Map Maker is not available in every country (it was expanded to the UK today).
Old Maps Online
An interesting development in the last couple of years in cartography has been the growth of crowd-sourcing for georeferencing and annotating digitised, high-resolution historic maps. In the UK the British Library has led the way with its historical maps Georeferencing Project. You can view all the crowd-sourced georeferenced maps already completed by the British Library on the Old Maps Online website.
A similar project has just been launched by Cornell Information Science with support from the University of Illinois at Urbana-Champaign and the University of Vienna. MapHub is an online application for exploring and annotating digitized, high-resolution historic maps. Using MapHub users are able to annotate historical maps and georeference locations on the historical maps using Google Maps and Google Earth.
The project has started with approximately 6000 public domain maps taken from the Library of Congress Historic Map division.
Galaxy Zoo - a citizen science project for classifying galaxies from the Sloan Digital Sky Survey