The Old 3dfx SLI Power
Scan Line Interleave (SLI) technology by 3dfx reached cult status during the heyday of Voodoo2, thanks to the near doubling of graphics power by purchasing a secondary Voodoo2 3D-accelerator card. Back then when graphics cards were all chugging along the PCI-bus and the topology of the PCI bus architecture is a multi master, multi target capability that allowed combining multiple PCI graphics cards to boost processing power without too much problems (though it's not as straightforward as it sounds). While such products were available in the workstation segment, the 3dfx Voodoo2 was the only such product then for the 3D game enthusiasts. Albeit expensive, it was still within the reach of hardcore gamers unlike higher-class workstation class graphics accelerators.
As for the Scan Line Interleave technology itself, this technology enabled each Voodoo2 card to render every other line of a frame. This meant that one card would render even-frame lines while the other renders odd-frame lines. While the technology was acceptable back then, it wasn't perfect as 'tearing' effects can be observed from scene to scene. In today's context where many gamers are highly drawn in to the finer graphics details, such effects are highly undesirable. Adding to the complexity are Full Screen Anti-Aliasing and Anisotropic Filtering, which make Scan Line Interleave an even more troublesome technology to implement.
The Return Of The SLI
The days of the Voodoo2 have long gone past and the PC industry has since adopted the AGP slot as the successor to PCI for graphics cards. AGP being a single master, single target protocol gave rise to the limitation of a single AGP slot on a typical motherboard. This is the reason why you don't see multiple AGP slot motherboards.
With the launch of PCI Express motherboards and PCI Express graphics cards, the limitations imposed by the AGP bus are no longer present (both in terms of communication and speed). Officially launched today on 29th June 2004, NVIDIA debuts SLI once again. Although NVIDIA retains the SLI marketing name (it now owns 3dfx rights and technologies), NVIDIA's SLI acronym stands for Scalable Link Interface and it has nothing to do with the Scan Line Interleaving technology from 3dfx. Using multiple PCI Express (PCIe) slots in conjunction with SLI, NVIDIA aims to scale graphics processing power across multiple GPUs.