The bright blue LED's on some cable modems I find over powering and I just put a small pin hole in some cut black electrical tape and put over the LED.
AND like adam9c1 I just face the several 16 port switches I use away from direct view.
I have in the past just put a small dab of ink from a black permanent marker pen on some led's
I have a small managed HP ProCurve 8-port switch which I know can be configured to dim the LEDs a lot. I don't remember if you can shut them off completely, though.