Celery salt is used for rimming glasses for a caesar or a bloody mary. It's used for pickling, fresh popcorn, and toasted pumpkin seeds. It's a useful condiment for chicken, turkey, casseroles, potato salad, deviled eggs, stir fry, meat marinades, chicken stock, meatloaf, and vegetable soup. It can be used to make things taste more celery-like and - strangely - at one pub, at least, it is used to rim glasses for hot cider.
I'm intrigued. I think I'll buy some - or at least some celery seeds, since, as some of you pointed out, it's just a mix of celery seeds and salt, and I already have plenty of salt in the house.
easterbunny: celery salt can be obtained in the UK. oursin lives in London and seems to be able to source it there, as does lazyknight in Bath.
Update: If you want to make your own celery salt (or celery pepper), buy whole celery seeds and grind them yourself. The powder tends to dry out quickly. They are also, as are all seeds, highly nutritious.