Check out this great fic written by amproof. It's a Numb3rs/24 crossover takíng place past Season 6 of 24. The prequel to this story :Always Travel In Pairs.
And for Ian Fans there are irena_adler's awesome Return of the Sniper-Fics, wonderfully placed in her Watson Universe. The first part you can find here in this community, the others in numbers_slash and in her LJ. Thanks!