IKEA hacker Stuart liked the IKEA Galant desk but wanted some extra storage, so he took some Rubbermaid wire storage and attached it to the underbelly of the desktop. A drawer was also added. Because most IKEA desks are made to accept additions, such as the SIGNUM cable organiser (which Stuart also used), you can pretty much drill anything into the base of the desktop.
All you really need to do to make this type of hack happen is figure out how you want to organise your stuff and attach the appropriate storage items. This is a nice, cheap DIY alternative to buying an additional drawer unit or filing cabinet.
If you need some inspiration, be sure to check out the IKEA Computer Desk Hack site for more photos and details.