Great news! Today VMware finally made the vSphere v5.0 Open Source code archives available for download.
Why is that important?
Since the release of VMware's ESXi 5.0 (Aug 24, 2011) many people are asking for the development of drivers for hardware devices that are not supported by ESXi 5.0 out-of-the-box.
ESXi device drivers are based on Linux device drivers (which lead to the persistent misunderstanding that ESXi itself is based on Linux), but the stock Linux driver code must be modified in a specific way to be compatible with ESXi.
With past versions of ESXi (up to 4.1) it was possible to study and reproduce these required modifications, because VMware published the source code of the ESXi device drivers (the original Linux code plus their modifications). The reason for this is that most Linux drivers are licensed under the GPL (General Public License), and the GPL requires that derived works are also published under the GPL and their source code is made freely available (aka the "Copyleft" principle).
Now, that VMware also published the Open Source Code of ESXi 5.0 (including the device drivers that it contains) it will be possible (or at least much easier) to develop custom ESXi 5.0 drivers for devices that are not officially supported by VMware.