TPM Muckracker has an itemized list of Ann Coulter's supposed plagiarism, and sorry to say, there's not much there
Coulter is a lot of things, but it doesn't look like plagiarism is one of them.
Update: Josh Marshall:
To me personally, some of the examples/accusations seem strained -- simply similar statements of the same basic facts. And sometimes there are only so many ways to describe one set of facts. In other cases the similarities of the wording strike me as hard to see as a coincidence. Especially when there seem to be multiple instances of similarities in the same column coming from the same source.
What these examples show is that Coulter is a lazy writer who rips off other people's research, but stealing someone else's arguments isn't "plagiarism". It's just being a lazy, unimaginative writer.