Gpsies.com is a website which lets you find and download tracks for walking, running, hiking, cycling etc. You can search for tracks by category, view the altitude profiles and download the tracks in a variety of formats. You can also upload and publish your own tracks on the site. It makes use of several map suppliers, including Openstreetmap.
As well as the website, there are a number of other interfaces to access the track data on the site. Mobile applications such as Maverick, Locus and Simple OSM are some examples, as well as desktop applications such as GpsPrune or GpsMaster.