Distinctive specialities are just that: specialities which are distinctive to an instructor, i.e. they do not appear in PADI standards. A distinctive speciality must, though, conform to PADI standards and be verified and approved by PADI. This is why the PADI www-site doesn't list distinctive specialities.
There's a number of ways an instructor can be allowed to teach a distinctive speciality. Other than what SubMariner writes, an instructor can follow a training course by a PADI Course Director, who also provides course outlines etc which the instructor must follow.
There are an impressive list of distinctive specialities out there, including at least one very good "underwater photography" specialities which go above and beyond the std. PADI UW-photo speciality, O2 administration, azimuth-rebreather, limited-viz-diving, buddying-for-handicapped-divers, compressor-handeling and so on.
It all depends on the instructor -- and as always, a good instructor will have good distinctive specialities and provide spectacular training in them, whereas a less-good instructor....well....you get the picture.