Before you start browsing the forums you should ask yourself just how many berries your LEDs should have, it could have a big effect on your next modding job.  You should also get in the habit of examining the fans that come with your coolers, for instance the Corsair H50 comes with a fan of questionable quality so you could swap it out with one that is more effective.  Modding can be done to just about every component in your machine, for instance why buy an expensive enclosure when you can make your own external HDD?  These types of projects are not without their risks, as this SSD owner found out.

If you are really looking for a challenge Linux is essentially a giant mod, where you can pick and choose which portions of the OS and programs you want to run and the gang over at our Linux Forum will help you roll your own OS.  Even Windows is moddable to a certain extent and you case modders can always hit up the forum where all the cool modders hang out.