Warning: this is an historic page kept as an archive and is outdated.
You can use OpenStreetMap’s Cycling layer (or many tools that use OpenStreetMap’s data), ProVelo’s maps or Luxembourg’s Geoportail. (The map sources differ slightly.)
Bicycle-friendly routes in Luxembourg
Here’s the second revision of the Luxembourg unofficial Cycling-friendly routes map. A few routes are still missing and will be added in the future, as usual.
You can also view the map on Google Maps here. I had to reduce track resolution, but we’ll fix this soon; other work is in progress.
A bicycle-friendly route is either an official cycling route, a rural road where only tractors and bicycles are allowed, some good quality tracks (mapped only when an area doesn’t have any other choice), very low traffic roads and rarely roads when they are convenient to merge two other sections, as long as traffic is not high.
I made the map with Viking, a free/open-source program for managing GPS data. You can download the work file here.
If anyone wants more detailed roads until I write a download and/or zoom script, please contact me, and I’ll send you my GPS log files (in GPX format).
You may want to visit GPSies, a community for vagabonds with their GPS.
Update in 2012
As an active OpenStreetMap contributor, I added all the missing ways, so they are now on the Cycle Map.

Hi there!
I would like to have a copy of your map of cycling roads in Luxembourg, if you be kind to send me one? (:
Kind regards, Ann Catrine
Dear Ann Catrine,
Thank you for your message, it was a nice trigger, I updated the map and made it bigger.
Best!
Tristan