Guichet.lu: “an information portal that simplifies your exchanges with the State and offers you quick and user-friendly access to all the information, procedures and services offered by Luxembourg public bodies.”
Delano Expat Guide: Can’t be linked directly, choose “Expat Guide” and then a subcategory from the menu.
Just Arrived: Website in French and English and printed guide which is published every two years (can be downloaded from the website).
This list is a work in progress. Please leave a comment if you know other “expat guide” websites which should be added.