This website allows you to view topo maps inside a Google maps interface. Quite cool actually but only available for US and Canada. (Piss-poor maps, eh!)
May be more useful in future when OS maps become openly available…
Moab in Utah
http://www.mappingsupport.com/p/gmap4.php?ll=38.575246,-109.546051&t=t2&z=15
Vancouver's North Shore
http://www.mappingsupport.com/p/gmap4.php?ll=49.35384,-123.053656&t=t2&z=15
In the top right hand corner you can switch between the usual Google views plus 'My Topo'
You could display GPX routes via Google Docs until Google changed the way their file upload process the other day. Hopefully the developer can work around that…
See also:
http://www.mappingsupport.com/p/gmap4.html