Here's something that serves almost no practical purpose and effects only a tiny fraction of enthusiasts, but yet is still interesting to see nonetheless. The crew at OCWorkbench got SLI working using a combination of modified drivers and PCI Express and AGP GeForce 6600 cards with a motherboard based on a ULi chipset.
Next up, SLI with PCI-based GeForce 6200s. (Just kidding. I made that last part up!)