I've been meaning to blog about this for a while now.
Jon Udell: LibraryLookup (Build your own bookmarklet)
What this little page does is create a bookmarklet for finding ISBNs (on for example, Amazon) and then tries to find the book at the library. Since I tend to use my Amazon Wishlist as "stuff that looks interesting, but I'll have to look at it later", I can now quickly export that list onto my account on the library and put it on hold. It's been quite useful for me (and has probably saved me some money that I would have otherwise spent of buying the books).
Here is the bookmarklet for San Jose Public Library.
It's great service. However, there is always a limit on the number of renewals for library books and there may not be enough time to finish a book before one exceeds the maximum of renewals patrons are allowed to have for their books. =)
According to my sister, the limit at the San Jose Public Library is 99 items, which is far more than I would ever check out at once. I think you're allowed to renew twice, extending the 3 week checkout period to 9 weeks.