Outside Casing Only
If all you need to do is open your casing, you want the Tri-Wing screwdriver, a specially designed tool that matches the screws used in Nintendo console casings.
Available on most well-respected Internet marketplaces, including eBay and Amazon, the Tri-Wing will get your Wii cover off in seconds with no damage to the screws (if properly used).
Light Internal Upgrading/Repair
If your needs extend beyond simply opening up your Wii console, you'll want to splurge on the dual set of Wii tools: the Tri-Wing and the mini screwdriver.
While you can open the case with the former, the mini screwdriver allows you to remove the Wii's internal shielding and casing for such components as the hard drive and the disk drive.
Complete System Repair
If you need unfettered access to every last component, circuit, mechanism and doodad inside your Wii, the 6-piece Tool Set is for you.
Comprised partially of the aforementioned Tri-Wing and mini screwdriver, the 6-piece Tool Set also includes three different sizes of hex-nut screwdrivers (T4, T5 and T3), as well as a tool designed to remove the odd star-shaped screw.