My clip tuner is a TuneTech Tunner TT-1000 and I lurve it! Carry it with me wherever I go.
Also, not clip-on, but pretty cool to know, if you have an iphone, there's an app called OmniTuner that is terribly sweet. It has a standard chromatic tuner, a clef tuner, and a frequency tuner (I don't really use the last two, but they are neat)
But sweetest of all, it has a fretboard tuner with many settings, you can look up banjo, ukulele (standard) ukulele (baritone), mandolin, a number of guitar settings, violins, etc. There is a picture of the fretboard of the instrument you choose and as the string you're playing comes into tune, the string glows from red to yellow to green (in-tune). Not only is it so cool and cute, it's also a great way to illustrate tuning to any noobs you are introducing the uke to. I used it to teach my 8 year old nephew how to tune his Hilo and it he picked up the concept much faster than when taught him with a traditional tuner. So, if you have an iphone, I highly recommend checking this app out.