I made an online tone generator based on the Firefox Audio API HTML5 Web Audio API. It’s basically a large logarithmic slider that allows real-time, smooth frequency changes. You can also fine-tune the frequency in 1 Hz increments, if necessary. (Update: You can also pick a music note from a list.)
There are other tone generators on the Web, but they are not as cool (if I do say so myself) and/or they require Java or Flash.
Warning: my generator only works on recent browsers which support the Web Audio API, such as Chrome and Firefox.
What can you use a tone generator for? You can do a science experiment with resonance, tune a musical instrument, test your new audio system (how low does it go?), test the limits of your hearing (I can hear absolutely nothing above 19,000 Hz, even at maximum volume), or figure out your tinnitus frequency to better target therapy.