In 2005, Oxford University Press published "The World Atlas of Language Structures" it is a 712 page hard-cover book and is available for £425,00. It comes with a CD that allows you to get a visual grasp where languages are spoken. It is filled to the brim with all kinds of information and it is considered an important work for people studying linguistics.
In a joint project several of the Max Planck Gesellschaft Institutes, published WALS online. It provides a rich resource and uses Google maps to indicate where what languages are spoken. The data is on the Internet and in order to be of value in a world where Open Access is increasingly considered to be essential, the data is available under a Creative Commons license.
For Wikipedians, the license will be disappointing; the CC-by-sa-nc does prevent the use of this data. WALS is considered a standard work and there are many references to the ISO-639-3 standard; check out Finnish for an example. However, many of the languages recognised in this Standard are missing, Stellingwerfs stl, Abau aau to name just two. I hope that this will be resolved as it will increase the value of WALS a lot.
Even when the use of this data is limited, there is a lot to learn from WALS. It integrates many of the facts known about many languages, it presents them in maps, and makes the best use of what Google maps has to offer. With this much information available it is fun to see what languages are spoken where. Wals on line is truly of interest and I heartily recommend it to people who share my interest in languages.