NH2112,
Your question certainly created a lot of dialog. Lots of fun stuff here. I'm starting to slow down but I have been bushwhacking for over 50 years. I find it is still much more fun to navigate with only a map and compass. Getting a feel for the lay of the land, using natural landmarks to guide you, visiting places that few do, you can't beat it. I've read through the responses, but I guess I'm still a little confused by your original question. Are you looking for on-line maps OR are you looking for a GPS application for your phone (that can use those on-line maps)? You have been getting answers to both these questions.
I still haven't bought a "real" GPS, but I have used them. They are quite the modern tool. Hiking GPS devices are no different than the GPS in your car. They will take you to exactly where you want to go. They can follow GPS tracks that other hikers have uploaded. They will guide you anywhere and also provide lots of other information. They are an amazing device but, if this is your only way of navigating in the backcountry and it fails for any reason, you now are on your own. I have always believed that anyone that hikes off trail should have competent map and compass skills. Plus, it's just fun.
That all being said, I have an Android phone. I use Backcountry Navigator. It accepts all the common maps including CalTopo. You can pre-download any needed maps. I have been using CalTopo because it is free. Last time I checked Backcountry Navigator was $10. It utilizes the GPS satellites, not cell towers so you put your phone on Airplane Mode while hiking thus saving power. I still don't use my phone for navigation, but it has been fun to find out my elevation gain and loss after some off-trail hikes.