MergedFB
MergeFB is an XFree86 driver feature for video cards that have multiple heads. It is similar to Xinerama in that it also creates one big virtual desktop out of multiple screens.
The difference is in the implementation: Xinerama creates multiple screens with multiple framebuffers and MergedFB, like the name says, creates one big framebuffer with multiple 'viewports'. The main advantage of this method, is that it supports hardware 3D acceleration on all heads and it's generally faster. This is similar to 'TwinView' used in the nVidiA drivers and the 'Big Desktop' in the ATI drivers.
Currently, this isn't supported in the drivers that come with the latest stable XFree86 release. Only in the MGA drivers from Matrox and the experimental open source drivers for the SiS and ATI RadeOn cards.