CiteSeer.
I spend most of my non-work hours there and it's horribly broken. Case in point, I just went there now to tally the most visible aspects of its brokenness, turns out it's down, again.
Yeah, CiteSeer is horribly broken. ArXiv is mostly for crackpot physics and has a tiny CS/EE index. What else? I refuse to pay ACM, Springer, or IEEE.
Google Scholar just .. looks "search enginey" and doesn't have the summary bar with citations, years, etc. Also, it heavily links to the above "resources" that I refuse to pay for.
CiteULike is just as lame as CiteSeer in terms of UI, but lacks the info-bar on the side, and adds crappy social elements.
CiteSeer gets the caching right though, which is why I keep going back to it.