I am guessing that at least some of you are already familiar with this tool, but some may not be: Google books. This tools allows you to search for and view pages from an astonishing number of books. Ihave found this especially helpful as I work on revisions for my dissertation. I come across a book that is referenced and I want to look at it, but my school's library does not have a copy. I could use Interlibrary Loan and wait for several days for the book to arrive. Or, I can search for it in Google Books, find the relevant section, and get what I need in a matter of seconds.
The only downside is that for copyright reasons some pages/sections of some books are omitted. So sometimes the section I want is not available. But in most cases it is. An additional advantage is that if I know that a book is available on Google books I do not need to take the hard copy with me when I travel, thus lightening the load.
If you are familiar with other helpful tools available on the web, feel free to leave them in the comments.