As an aviation artist and 3D modeler I am often in need of 3 view drawings, flight manuals and reference photos. I had been using scribed.com to find and download aviation related books and drawings.
However their inventory was limited and not aviation specific. I then found a great site called Avialogs http://www.avialogs.com/.
Avialogs is a not-for-profit organization dedicated to preserve our aeronautic heritage and to build an online aviation library. Their site contains technical documents, newspapers, books related to the various fields of aircraft.
The site is fairly new but rapidly growing. With access to over 6500 documents the cost of a subscription is dirt cheap for wealth of material available for download, especially compared to other document download sites.
The site is well build and very professionally setup. Navigation is easy and finding what you are looking for is a snap. Documents are well scanned and categorized as well and the download speeds are good.
Since their site is not for profit they can always use your help. Please consider helping in any way you can. You can donate here: http://www.indiegogo.com/projects/avialogs-com-one-aviation-library