I am happy to announce the σ26, a negative version of the σ25 PSU board. The σ26 has the same size and very similar layout as the σ25, and supports the optional σ24 EI-30 transformer board. The supported voltage regulator is the 79xx, LM320, LM2990, or the σ79. Most polarized parts on the σ26 are to be mounted in reversed orientation compared to the σ25 (the bridge rectifier, electrolytic capacitors, diode and LED).
Since σ26 is essentially the same as the σ25 (except to accommodate the negative voltage regulator's different pin-out, and reversed voltage polarities), there is no development forum thread for it.
σ26 boards are now being manufactured. The release date is in approximately 3 weeks from now, around the same time σ78 and σ79 voltage regulators should also become available.
The official σ26 website is now up. Have a look.