Is one allowed to use/inhale Snuff on Yom Kippur? Are there any sources that explicitly say it's ok?
Even if one says it's "technically" not eating, could there be any problems with using it on Yom Kippur? (ie not on the spirit of the day etc)
R. Moshe Chagiz in his Leket HaKemach (Orach Chayyim Hilchot Tish'ah BeAv uShear Ta'aniyot) writes explicitly that inhaling snuff on Yom Kippur is permitted, as there is no prohibition against smelling pleasant scents on Yom Kippur. (Interestingly, he notes that on Tish'ah BeAv he was particular to only use plain snuff, without a pleasant smell.)
However, he goes on to say that it should not be done publicly in the synagogue, as it leads to many uneducated people acting in a lighthearted and inappropriate manner in the synagogue. In fact, he writes that if he had the ability to do so, he would fine those using snuff inside the synagogue.
Supplementing what JoelK mentioned, I see that Leket Hakemach mentions that it is common to smell besamim on Yom Kippur in order to fulfill the reciting of 100 Brachot during the day. (Thanks, JoelK for the reference to this.)
Each year, my shul gabbai brings in specially marked bottles of spices (I never knew that star anise smells so good!) donated by someone who puts a label on the bottle saying "in memory of ..". So, each time someone makes a bracha before smelling, not only does that person fulfill the bracha requirement, but both hat person's as well as the deceased's neshamot get an "aliyah".
However, Ba'er Hetev commentary on O.C. 46:3 specifically addresses the problem of getting in 100 brachot on Yom Kippur and advises against sweet smelling spcies on Yom Kippur, thinking that doing so is a violation of inuy (affliction) on Yom Kippur. He therefore suggests that when one answers Amen to the shatz's brachot it is as if he made the bracha, himself. Thus, he would have accomplished 100 brachot during the day, and not need to rely on the besamim , snuff, etc. (With 7 brachot in each Amidah and having 5 silent ones (35 total) and 4 from Shat"z (28 total) making 63 on Amidah alone - 14 on Birkot Hatorah, making 77 total, add Birkot Shema and Birkot Hashachar, etc. - this could be quite easily accomplished.)