Google Scholar provides a simple way to broadly search for scholarly literature. From one place, you can search across many disciplines and sources: peer-reviewed papers, theses, books, abstracts and articles, from academic publishers, professional societies, preprint repositories, universities and other scholarly organizations. Google Scholar helps you identify the most relevant research across the world of scholarly research.I remember trying Google Scholar when it was first released. At the time, I didn't find it all that helpful and I more or less forgot about it. Recently, amidst the flurry of term-end papers, I came across it again and I must say it seems considerably more useful now. Classics majors will be happy to know that it searches JSTOR now. (JSTOR is a journal index that includes quite a number of Classics journals - if you are a Classics major and have never used JSTOR, go check it out! You can probably get full-text access to articles online through your university library.)
Speaking of libraries, one of Google Scholar's spiffy features is the ability to set your library preference (visit the preferences page and check out the 'Library Links' section). Pick your university's library and you'll get links next to search results (e.g. 'Get it @ Oxford'). This facilitates finding the book or article at your library (or getting online access through your library login).
Google classifies Scholar as 'beta', and rightly so - it still has a long way to go... (I sure hope it gets there!) Linked with Scholar is their full-text Book Search tool. It is also in beta, but is already quite useful (and will only get better as they add more books.)