I've got Hakko FX-888 as my main working tool. Great tool, one thing is annoying though - you can't tell at a glance if it's on or off. I decided I'll hack it as Dave brilliantly suggested in this post: EEVblog #242 – Hakko FX-888 Soldering Iron Hack. However I didn't check before ordering and got myself a bunch of common cathode LEDs. Gosh, had to think myself :-)
OK, just in case you'll need I'll save you some minutes. Here's the schematics:
You'll also have to short the 2k2 resistor on PCB, or replace it with 0 Ohm jumper. Have a look at the video Dave posted for an idea on how to mount this within Hakko body (heat glue gun of course).