I'm not sure if you're looking for a free web based tool in particular, but I just got a program called TopoFusion. It's a desktop (PC) app. Like you, I was looking for a program that would allow me to do trail planning. Most importantly for me, I wanted to use Mytopo maps (the greatest IMHO) and I wanted to be able to download gpx files etc. so I could plot everyone's routes and then plan mine accordingly. The last thing for me, I just bought one of the Garmin Oregon 450's on sale at REI last week and I really wanted to get those mytopo maps and tracks onto my Garmin. I'm not so interested in tracking how far I've gone and all that stuff, I'm more interested in "where am I" as it relates to which trail I'm supposed to be on and where to make the next turn!
Last year going up the MT was a great example; It's 3am, dark, I'm working hard going up and all I need is a quick glance at my GPS showing my dot on the trail route to know I'm going the right way. Taking that guesswork out of the route is invaluable to me.
So anyway, just thought I'd mention TopoFusion, it's a little pricey but it does those 3 things I mentioned above which more than makes sense for me.
