There are any number of fine online map sites out there, but the collection curated by librarians at the Perry-Castañeda Library at the University of Texas-Austin is my favorite.
The main reason is how efficiently they have indexed their collection. On its home page, for example, the library features links to the most current and newsworthy maps. Right now, for example, they have links to five maps showing the 13 November Paris attacks, as well as maps of Iraq, Syria, and the Ukraine.
Further down the home page the collection links to a series of online maps of general interest.
The home page also links to another page that includes a rich variety of historical maps.
You could easily spend fifty enjoyable lifetimes exploring PCL's collection. So set a time limit, use blinders, get what you need, and then close your browser window until your next visit.