ETHoS e-theses online service [ http://ethos.bl.uk ] ETHoS is a National database of UK Doctoral theses maintained by the British Library. Many theses are downloadable as full text. ETHoS is free to use. Access to full text requires an individual registration.
BASE Bielefeld Academic Search Engine [ https://www.base-search.net ] BASE includes masters and doctoral level theses found in c5000 sources around the world. To search for theses go to Advanced Search and select Thesis as Document Type or one of the subsets.
WorldCat [ http://www.worldcat.org ]
WorldCat includes MA/MSc as well as MPhil/PhDs with wide geographical and chronological coverage. To search use Advanced Search select Content and Thesis/dissertations. Use Keywords and Subjects to search for relevant topics.