Ordinary people like you and me can sometimes accomplish extraordinary things.
La gente corriente como tú y yo a veces pueden hacer cosas extraordinarias
The closest "ordinary people" translation I can think of is "gente corriente". You could also say "gente normal", "gente de la calle" (lit. street people), "los ciudadanos de a pie" (lit. walking citizens), "el pueblo" (the people).
I bought a run-of-the-mill guitar; I didn't want to spend a lot of money on something special.
Me compré una guitarra normalita... (that's probably pretty Spain-centric and not very formal)
You could also say "una guitarra corriente", but it isn't very natural sounding. There's also "una guitarra del montón" (lit. a guitar from the heap, i.e. a random one from a bunch of guitars), but I wouldn't really use it for a guitar.
He's just an average Joe.
Es un tipo de la calle.
There's "es un Don Nadie", but that's more like "he's a Nobody"; or "Es un cualquiera"... both are slightly derogatory.
How did a regular guy like him wind up being a movie star?
¿Como un tío normal como él acabó siendo una estrella de cine?
Or "tipo corriente", "tío de la calle"... I consider "guy" to be a bit slang-y, so I'd use "tío" (Spain's "dude", lit. uncle).