Oh wow. My DS has a dairy allergy and is invited to parties, although I don't know how many have excluded him. There's no way a child should be excluded because of a medical issue!

For me, depends how close DS is to the child, or I am to the parents. Books or jigsaws usually go down well for either gender (about £10), but for a little girl DS didn't even know from his new class at school, under £5 for some fairy wings, fairy wand and a card. Cheap science toys are great for slightly older children. I bought DS a solar-powered grasshopper you have to make yourself for under £5 (all electrical componants etc provided in box).

I try to avoid high-heeled play shoes, barbies (etc) or attack toys, including things like Jedi/X-Men stuff. Does rule out quite a bit, but only my SiL has ever complained (to my face, at least) about her children being given books. Unless I know the mother/child in question adores the stuff.