What other Enumerable extension methods would you like to see?

A few questions on Stack Overflow have suggested to me that there might be some bits missing from LINQ to Objects. There's the idea of a "zip" operator, which pairs up two sequences, for instance. Or the ability to apply the set operators with projections, e.g. DistinctBy, IntersectBy etc (mirroring OrderBy). They're easy to implement, but it would be nice to get a list of what people would like to see.

So, what's missing?

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