Alternatives I've tried.. OpenStreetMap's maps all seem to be road maps. The OpenCycleMap does include terrain, but then it has a lot of other high contrast features on top that make it unsuitable for my needs. USGS has some terrain tiles at the National Map, but they're unstyled and lack any descriptive features. I know it's possible to generate and serve my own tileset using a free datasource like SRTM, but that's an awful lot of work to design and operate.
I'm able to pay a modest cost for bandwidth; mostly I want to avoid spending the time generating my own tileset. Terrain maps seem like a common need, I'm surprised I haven't found one!
Edit since posting this question I've found a few more options. Microsoft's Bing Maps include terrain tiles; with somewhat flexible, but limited terms of service. GeoiQ/GeoCommons has the Acetate tiles which are specifically designed for my purpose.
Edit 3 MapBox also now has a terrain layer. I'm unclear about the licensing, but the company is generally open source friendly.