I'm looking for a space-efficient data structure that holds sets (no repetition) of wordsize elements and supports fast insertion (amortized O(1)). By "space-efficient" I mean, ideally, $n + o(n)$ words to store $n$ elements.
Being a set is an important part of the question: if each element is added $\log n$ times the space used can't be $n\log n$.
The structure should also support listing its elements (reasonable efficiently); any sane structure should have no trouble here. (Fast membership queries are a plus.)