Google Scholar is a great resource when I'm doing a literature search. However, sometimes I find it frustrating to use. I want facets with year, author, number of citations (or similar), conference, and organization for starters.
Microsoft Libra has some facets but doesn't carry it all the way through. For example, it's not available when you are browsing citing papers. It's coverage also seems to be missing papers/citations. CiteSeerX isn't any better.
And our venerable CIIR Publication search would also benefit from some sprucing up.
Honestly, most often I use regular Google Web Search which has the best overall coverage even if I'm limited to using keyword search.
What do you use to find papers? What do you like and dislike? I want to hear what other people use.