The Xbox 360 Marketplace has announced the end of its existence. We have provided all of the information available so far on the subject here.
The official Xbox Support Page provided the news. The Xbox 360 Marketplace will come to an end in May, although Microsoft explicitly states that you must have purchased your games and DLCs by then. Games that have already been purchased may of course be downloaded again after that. Only yesterday Sony announced the ending of the PlayStation Plus Collection.
The Xbox 360 was launched on November 22, 2005.