[Lord Syslox of Emyn Beverli]

I know this is an old thread, and a much debated issue, but no one seems to have mentioned the quote i have come across.

Regardless what finger Frodo wore the Ring when it was bitten off by gollum, there is proof that he wore it on his first finger (next to his thumb) of his left hand.

In "A Knife in the Dark", right before the Nazgul attack, this quote is obvious:

"He shut his eyes and struggled for a while; but resistance became unbearable, and at last he slowly drew out the chain, and slipped the Ring on the forefinger of his left hand."

This is on page 257 of my copy of FotR.
The dictionary states "forefinger" as the first finger after the thumb (also called the index finger).

There you have it. It doesn't prove whether Frodo is right or left handed (where he puts the ring is upto personal preference), but it does prove specifically where he had the ring once.