for the USA and Alaska there is a free one

have a look at SkyVector
http://skyvector.com/

if you click on charts then Enroute High then you can select the parts of the usa you would like

Ive also found this site http://www.fltplan.com/ which covers USA and Canada, some stuff you have to register for and maybe pay but there is other stuff there that is free to use by the looks of it