Another type of chalcid wasp, these I think are probably Torymids, possibly Torymus sp. (Hymenoptera: Torymidae), but I am not sure of that. The females use their long ovipositors to lay their eggs on the larvae at the centre of oak galls (see here for a photo).
The males seemed a bit smaller than the females, even allowing for their lack of ovipositor:
[Note added 2010-02-04: I had assumed that, because this wasp was close to the Torymid wasp in the first two photos, that it must be the same species. However, in a comment below, Hannes Baur suggests that it is more likely a female Eulophid wasp of genus Sympiesis, possibly S. sericeicornis.]
Photos taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2009-10-08.