Google Scholar
I must say, having been a long time CiteSeer user, Google Scholar is a real breath of fresh air. It is yet another academic search interface, although this time its done right (unlike Rexa which is waaaay too inaccurate). Its a great interface and you can actually find whatever your looking for, its quite amazing!
Library support is on the way, too. At the moment though, I could only find the National Library of Australia and Deakin University, but the level of integration is very promising.
Tip: Maintaining your BibTeX reference database can be a pain sometimes. When using Google Scholar, make sure you enable BibTeX export in the preferences, it will save you heaps of time. Even when you already have the PDF, it is easy to do quick search, click “Import into BibTeX” then the entry into copy & paste into your .bib file. There is a slight bug with the field “authors” instead of “author” but that is easy to fix on the fly.