I have a fan set up which turns on when a button is pressed, but it starts and stops really fast. Is there a way to make the fan gradually speed up and then do the same when it is stopping, gradually slows down.

I THINK you can increase the friction key value of the func_rotating, but I'm not sure if that would work.

I've been wanting a similar effect with a floating platform.

Doesn't a func_rotating have a speed input? You could use a relay to eventually slow down the fan to 0.

You could use a math_counter to control the speed of the fan.
Use it with the output "OutValue fan SetSpeed <empty>".
Via a logic_timer you can add or subtract a amount of speed each time it fires it's output OnTimer.
For example: "OnTimer math_counter add 1".

I have the fan speeding up at start and slowing down at the end and its so simple to do. Go to flags and check the Acc/Dcc flag and just change the friction to suit your needs.