Soft SPL allows the flashing of custom ROMs (any ROM) to the HD2, though unlike "hard" SPL it is not permanent. It must be used each time the device is flashed.
Since its release this morning, there have been numerous ROM releases, most of which can be got from XDA-Developers.
You can get SoftSPL from here, where there is a guide on how to use it.
Note, we can't be held responsible for any damage to your device, and it's a beta designed for the more advanced user.