If you're writing a fanfic set in Middle-earth, and particularly if you intend to drop someone from modern Earth into the story, you may well have need of 'genuine' names for the main characters. Calling Maura Labingi 'Frodo' would be equivalent to meeting Julius Caesar and talking about 'France' - it's a wildly anachronistic translation which will get you, at best, blank stares.
It is in part to address this issue that I began the Detranslation Project, and this page chronicles the results. In the lists below, names listed with an asterisk before them are attested by Tolkien; other names are my own reconstructions.
Frodo Baggins - *Maura Labingi
Samwise Gamgee - *Banazīr Galbasi, called *Ban. His surname was usually corrupted as *Galpsi
Gandalf - called *Mithrandir by the Elves
Aragorn - *Aragorn is a Sindarin name
Boromir - *Boromir is a Sindarin name, and he has no other
Legolas - again, *Legolas is an untranslated name (Silvan Elvish, rather than Sindarin)
Gimli - a Norse name meaning 'fire', translating a Dalish original. My suggested translation, based on the Adunaic word for 'star', is... Gimli.
Meriadoc Brandybuck - *Kalimac Brandagamba, called *Kali
Peregrin Took - *Razanur Tūk, called *Razar or even *Raz
Since the Fellowship aren't the only people in Middle-earth, I've also translated some other names. The following list excludes anyone already named in Sindarin, Quenya, Adunaic, or any other language not translated by Tolkien. It includes Westron, Dalish/Northern Mannish, and Rohirric.
Bilbo Baggins - *Bilba Labingi
Lobelia Sackville-Baggins - *Hamanullas apparently means 'Lobelia' (the flower)
Rosie Cotton - *Hlothran is the surname 'Cotton'. There is a generic Adunaic word inzil, 'flower', which could give Westron Inril. Alternately, Sindarin meril means 'rose'; it could lead to the full Westron name Mirīl Hlothran.
And one final list... places, and other things in need of names.
The Shire - *Sūzat
Bag-End - *Laban-neg
Brandywine (River) - *Branda-nīn, jokingly corrupted as *Bralda-hīm, 'heady ale'
Rivendell - in addition to the archaic Sindarin *Imladris, we also have Westron *Karningul
Westron (!) - *Adūni means 'Westron', and *Sōval Phārė means 'Common Speech'