The Return of the King
Butterbur is the innkeeper at the Prancing Pony in Bree. It is Butterbur who first warns the hobbits that they will find strange things going on in the Shire; he also confesses that Bree has been seeing a lot of trouble from local thieves over the last year. You can imagine, given Butterbur's suspicion of "Strider" in The Fellowship of the Ring, that he is shocked to hear that Strider is now the king of Gondor.
In the second half of Book Six, we are meeting old acquaintances all over the place. Bill Ferny is the nasty piece of work who sold out Frodo to the Nazgûl in The Fellowship of the Ring. Now he's back as gatekeeper to the Shire, but the four hobbits so frighten him with their fine armor and their swords that he runs away, never to be seen again.
Bill the Pony
Sam's beloved pony from The Fellowship of the Ring; Sam discovers that Bill made his way back to Bree from Moria against all odds. Sam is happy to take Bill back with him to the Shire.
Bob and Nob
Butterbur's two servants; we see a bit more of them in The Fellowship of the Ring.
Harry the Bree gatekeeper is a very minor character in The Fellowship of the Ring; he reappears only in conversation in The Return of the King, when Gandalf asks the current gatekeeper where Harry is, and the gatekeeper replies only: "Gone [...] But you'd best ask Barliman" (6.7.12). Butterbur tells Gandalf that Harry disappeared with "the strangers" (6.7.29)—the thieves who came to Bree during the bad year, when Gandalf and his friends were struggling with the Ring Quest.
Mat Heathertoes, Rowlie Appledore, Tom Pickthorn, Willie Banks, "one of the Underhills from Staddle"
These are the five victims murdered in the battle to drive thieves from Bree some time during the year that Gandalf and Company were on their various quests. Butterbur reports their names to Gandalf, Frodo, Merry, Pippin, and Sam when they come to stay at The Prancing Pony on their way back to the Shire. Three of them were men and two were hobbits.