besides Pots is there any computer program that can interact with ARduino and allow me to control LED brightness with a slider or key stroke?
Many programs can ...
in essence you should define a command set that goes between Arduino and PC and back which defines the values to be displayed. zillion examples on the forum.
How many LEDs?
If just a few connect it and a resistor to a PWM output and use analogWrite(),
if lots then get some chips like a TLC5940.