hypermegaglobal.net

Everyone knows Google Maps (which BTW now allows you to add your local business for free), but there's more:

1. Microsoft's Bing Maps can show you Luxembourg in an awesome "bird's eye" aerial view with multiple angles.
2. User-generated OpenStreetMap is the best site to find hiking and biking trails which are not displayed on Google or Bing.
3. GR-Atlas visualizes a range of topics from the greater region (found on wort.lu).
4. Luxmap displays "relevant local village and business information" (currently only for Dudelange, Hesperange and Roeser), though I doubt their attempt to get businesses to pay for a listing will be very successful.
5. Update September 1st, 2010: topograhpie.lu is Luxembourg city's urban information map.
6. Update February 23rd, 2011: map.geoportal.lu has the newest 2010 aerial photos (better resolution than Google Maps)!