Forever - nominated for Best Multiple Crossover (it's Buffyverse / Highlander / about four other crossovers, most notably Veronica Mars and Six Feet Under)
The previous ones are
Best non-Buffyverse crossover - The Return
Best Highlander crossover - Grounds For Divorce
Best Multiple Crossover - Kafkaesque (yes, this does mean I have two stories in the same category, possibly reducing the chance of either winning, but never mind)
Best Movie Crossover and Best of British - Deeds of Maidenly Unkindness
Voting doesn't start until 16th August, so it's possible that there will be more.