Multiple brands make the clip on guitar tuner and they come in a wide range of costs and styles. Check out all of them through the following link:
Browse all clip on tuner options here.

The clip on guitar tuner does exactly what the name suggests. It clips on to the end of the guitar and gives a read out of frequencies on the display just like a typical guitar tuner. I play guitar too and it is annoying to try to balance a regular tuner on my knee while I attempt to tune up.

One of the most popular, and highly rated clip on guitar tuner makers is the Snark brand. They have a few versions including the SN-1 and SN-2. The Snark guitar tuner shown on this page is the SN-1.

The Snark clip on tuners clip to the head of the guitar and can be rotated and positioned for your eyesight. The display is bright and easily readable for those with poor eyes. Very sensitive to give accurate readings, and tons of great customer reviews which you can read for yourself.
Read More About Them or Buy Them Here!

Pros: 

Accurate, and easy to read. Relatively low cost. No more balancing on the knee. Tons of good reviews, and a top seller. Positions however you need it. The rubber on the clip is designed to not wreck the guitar.

Cons: 

I have read through some negative reviews which you can check out for yourself. One thing that seemed to come up was that you need to be gentle with rotating the position so you don’t break it.

More Clip on Guitar Tuner options
Clickable Images

This is a more updated model of the Snark clip on guitar tuners like the ones discussed above. They call this the Snark SN-5. The display is full color, it rotates 36 degrees for maximum visibility, and it has a wide frequency range to handle both guitars and bass guitars. This one has a sturdier clip and a more prominent display as you can see in the image.