Here's a very useful, and most importantly free, source for online mapping. Just click on the link below, then click on "Make a Map." There are many different Basemaps to choose from, including topographic features and aerial imagery. You can also add additional data layers or create your own layers. Do yourself a favor and add this site to your favorites.
http://www.arcgis.com/home/
http://www.arcgis.com/home/