I made an online tone generator based on the Firefox 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.
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 Firefox. Due to a bug in the Linux version of Firefox, it doesn’t work on Linux.
What can you use a tone generator for? You can do a science experiment with resonance, tune a musical instrument, test the limits of your hearing (I can hear nothing above 19,000 Hz, even at maximum volume), or figure out your tinnitus frequency to better target therapy.