Capo[]
Capo
A small device used to raise the overall pitch of the guitar by pressing on all six strings of a given fret simultaneously. The full name is a Capotasto.
Plectrum (Picks)[]
Picks
Guitar Picks are used all the time. It really depends what kind of a guitar player you are, but unless your into fingerstyle or simply don't like guitar picks - then you'll be using a pick. They are great for providing a solid, vibrant sound. Picks also make it easier to hit individual strings. They aren't as fat as your fingers, so they can slip in between strings very easily and hit the strings.
Electronic Tuner[]
Tuner
For first time players, tuning by ear isn't gonna happen. It can also be tough to tune using a tuning fork, piano, or other method then electronically. This is by far the easiest way to tune - the device indicates the note and how far off tune you are.
Tuning Fork[]
Tuning forks (left) can be used to accurately tune a guitar. For beginners they are more challenging.
Slide[]
Slide being used
A slide is a glass, stainless steel, brass, or chrome tube you put you finger in. The slide is pressed against the strings — lightly, so as not to touch the strings to the fretboard, and parallel to the frets. The pitch of the strings can then be continuously varied by moving the slide up and down the neck. The usual limitation in fretted guitar playing of twelve pitches per octave does not apply. Indeed, in pure slide guitar playing the frets serve no purpose, other than as a visual reference. The technique lends itself to glissandi (swoops up or down to a note); in addition it has the ability to evoke sounds of the human voice, crying, sighing or weeping, or natural noises. Another strength of the technique is its vibrato, which is easily achieved by oscillating the hand so that the slide goes quickly back and forth.