Bit Commet has done build into it. It's rather easy to use, but rather limited as you only have HIGH, LOW and OFF.
Azureus has a third party plug-in callled Speed Scheduler that will allow you do to the same thing. It might take a little more time to program, however you have find tune to what speed you want according ot the time and day of the week. It's not hard to use, however it's a lot more detailed.
Both programs are extremely useful. Especially considering that my Off Peak Download time starts at 3 am in the morning.