I surprisingly got fewer answers than I expected. The most noteworthy response was from Ruchir Godura, who rattled a whole set of possibilities. I cannot prove that this is the exhaustive list, but here are the ones I have come across – 6 of them:

99 + (9/9)

99 + (.9/.9)

(9 + (9 * 9))/ .9

(99 – 9) / .9

(9/.9)*(9/.9) (and many other permutations of the same answer exist)

(99/.99)

Hope you enjoyed the puzzle!