The MIDI keyboard is the staple of every band that wants an analog synth/vocoder hook in their performance. It doesn't hurt that it's tiny and looks vintage--therefore, everyone from French electronic duo Justice, to Canadian experimentalists Crystal Castles to mainstream Kings of Leon and art rockers Yeah Yeah Yeahs use it to death.
Why it's overused: There are only so many presets you can use.
The banjo is a fine instrument with a long musical heritage, and it features on much of the best folk, country and bluegrass recordings of the past century. Unfortunately, this heritage, along with the its twangy, plaintive sound, has inspired many an indie rocker (Modest Mouse, Sufjan Stevens and Iron and Wine) to employ it in order to attempt to add a down-home flavor to songs that didn't really need it to begin with.
Why it's overused: In the wrong context, the sound of a banjo sticks out like a sore thumb.
This is a fairly new instrument to the indie scene. The melodica in its current form was only invented about 50 years ago, and it's a somewhat awkward-looking amalgam of wind instruments and with accordion-style keys. Hipper-than-thou bands like Gorillaz, Franz Ferdinand and Clinic have broken out the melodica on stage and on record.
Why it's overused: No matter how cool you are, you look like an idiot playing the melodica.
A descendent of the lute, the modern mandolin has been around since the 18th Century. prized for its of its distinct, trebly tone, eclectic musicians like Peter Buck of R.E.M. and Jack White have employed this small stringed wonder on record.
Why it's overused: Unless you're a musician on the level of a Peter Buck or Jack White, you should master the guitar first.
Renowned for being the largest and lowest-pitched brass instrument, the tuba can usually be found in large orchestras or marching bands. Nevertheless, some intrepid (or foolhardy) artists have attempted to add the tuba to rock, like Jellyfish, XTC and Austin band Drums and Tuba.
Why it's overused: Even high school band geeks know the tuba is not cool.
Technically a keyboard instrument, the accordion often appears in the folk musics of Central and Eastern Europe. It had its heyday in popular music in the mid-20th Century, but never really infiltrated rock, outside of "Weird Al" Yankovic's pop parodies. Krist Novoselic changed all that with Nirvana's MTV Unplugged performance of "Jesus Doesn't Want Me For A Sunbeam," however. Since then, countless rock musicians have added the mellifluous sounds of "the box" to their recordings.
Why it's overused: Using an accordion is OK occasionally, but it's really no fun at all to watch someone play it. It just looks ridiculous.