I just noticed that Rishon Rishon is the #1 Google search answer for ә. (Odd, since I think that's the only post where I use the character.) It's a schwa, which comes from the Hebrew word sh'va' (in my usual orthography). In Hebrew, sh'va' (שְׁוָא) means something like "nothing", and is an epenthetic vowel, but in linguistics it is this vowel, which is the most common vowel in English - it is the "e" in "the" and heard at the beginning and end of "America".
UPDATE: I know why. I must have inadvertently chosen the Cyrillic schwa for that post instead of the IPA schwa. It's only used in "Kazakh, Bashkir, Tatar and other languages of the ex-USSR" - not too common on the web, evidently.