I will not mention any cases, cables or straps, which are a must — except for the Slider Dual Shoulder Strap. It's a strap dedicated to those who have fragile vertebras, and allows you to evenly distribute the weight of your instrument on both shoulders.
For those of you that have a tendency to lose your guitar in the middle of a solo, a very effective solution consists of buying strap locks. They can be installed in less than ten minutes on your guitar and strap, and will allow you to jump on stage without having to think twice.
A definite classic, especially for blues and rock, is the slide. Although you could always use a beer bottle, lighter or any other rounded device (used for the consumption of non-toxic or toxic substances) instead, these small tubes prove to be more practical due to their ergonomic design.
Another widely used accessory is the capo, which allows you to automatically transpose your chords in no time. There are many models available, from a simple clip to the Spider XXL, which allows you to set every string individually.
Guitar multi-tools are like "Swiss Army Knives" for guitarists, and feature different guitar-specific tools within a single unit. Guitarists will find them extremely handy, particularly those of you who own several instruments.
For your hands
In order to save all your energy for your playing, several brands offer diverse tools to help you change strings. The principle behind the string winder is very simple: make string changing easier and faster. From the most basic ones, costing around $3, to the most sophisticated, like the electric Power Peg, they can come in very handy if you need to change lots of strings, and save you precious time.
If, on the contrary, you would like to exercise your hands, you can use some devices conceived specifically with guitar players in mind. Once again, there are many different models, like the traditional Gripmaster or the Varigrip, with variable spring tension.
For your picks
Are you tired of losing your pick on stage and having to get one out of your pants pocket? Then the pick holder is for you. From the simplest and cheapest models to pick cases with special designs, these small objects can be glued to your guitar or clipped on to your belt or strap.
If your problem is not losing picks, but rather wearing them out too fast, you can make them yourself with the Pickmaster Plectrum Punch. An old credit card or any other plastic object is enough to make your own custom picks.
For acoustic guitars
Here you have two accessories conceived specifically for acoustic guitar players.
Ergoplay allows you to put you guitar in a "classical" position with the aid of suction cups.
For instruments that show signs of dryness, a guitar humidifier placed inside the sound hole ought to be enough to provide them a reasonable moisture. A model with hygrometer will allow you to control the amount of moisture in real time.
And, since I'm in a good mood, here's an eleventh one! Fans of pedal steel guitar (or lap steel or Dobro) who don't have the money to buy one, will find in the Capo/Slide Converter a simple solution that increases the height of the strings and gives you classic capo functionality. And you could also use a tone bar (oops, that makes twelve!) for an even more authentic sound.
Obviously, this list is by no means exhaustive, so don't hesitate to post your own finds and suggestions in the comments below!