Thanks for visiting Nikita! After 2½ years, Nikita closed on November 14th, 2008.
I've enjoyed making this service available and I'm glad so many people found it useful. Nikita was popular, but not popular enough to justify continued maintenance and upkeep.
The Wed Design Group hosts a validator that crawls multiple pages, but the "validate entire site" option limits itself to 100 URLs. As far as I'm aware, that's the only service online that's at all similar to Nikita.