All the following resources are being kept very current, enjoy.
1) Listing of unique (see site for details) membrane proteins deposited in the PDB with references by Stephen H. White
-despite my best efforts, I could not find any additional websites relating to membrane protein structure that were not already listed on his site, great resource
2) Membrane proteins of known structure, crystallization conditions and key references by Hartmut Michel
3) Membrane Protein Data Bank by Martin Caffrey
-membrane protein statistics page is really neat, want to know the structures in the PDB that were crystallized at pH 5.6?
4) Bilayer Insertion of Membrane Proteins by University of Oxford
-contains a number of cool simulation results (click on the image of the structure)
5) Protein Data Bank of Transmembrane Proteins by the Institute of Enzymology
-allows one to search by the number of transmembrane segments