However you can change it fairly easily by changing the prescaler, particularly for Timers 1 and 2. You could do it for Timer 0, but that would change the rate at which the millis() figure updates.
Also if you want other frequencies and are prepared to dedicate a whole timer to it you can effectively select one from a wide range. I have a table on this page which shows the various frequencies you can achieve. However you have to get a bit closer to the hardware to do that, although there are some libraries that will help with that.