viaLibri: Search Multiple Libraries

This is a fantastic tool!
http://www.vialibri.net/library_search.php
Search 10 different union cataloges and over 12 rare book and special collection libraries at once!

Libraries you can search:

• Public version of WORLDCAT
• COPAC
• KVK
• ICCU
• CCPB
• ESTC
• ISTC
• STCN
• VD17
• artlibraries.net