Google's "My places" is great for showing lots of traces (in my case, paths representing bike rides I've done). But it only shows 20 or so per page.
I'd really like a map (a statically rendered .png would be ok - but prefer dynamic online) with 1-200 traces highlighted over a base map. I'm happy to use online or offline tools (preferably Windows, Mac ok, last resort Linux) to achieve this, but the fewer steps and unfamiliar technologies the better. (I'm not really familiar with any offline GIS tools.)
Lots of other sites allow you to upload traces, which they'll display in a list - but I haven't found any that will actually render them all simultaneously.