Microsoft Academic Search
Microsoft recently revealed the beta for their academic search - an attempt to compete with Google Scholar, which has been around for a year or so (no, I had a look, Yahoo is not yet offering something similar).
I use Google Scholar quite a lot, but immediately liked the interface of "Windows Live Academic Search" more. Their interface offers one critical feature I've been missing at Google: a way to directly access the bibtex data of the papers I'm looking at; with Google Scholar I often needed to do another web search to find the data needed to correctly cite a paper. On the other hand - at least in my area of Computer Science - Google Scholar seems to index a lot more papers, which in the end of the day is still the most important feature for a search engine.