Shadow nibs - broad nibs that have had a groove ground in their tip, usually asymmetrically - are fun to play with and also very good learning tools for anyone working on broad-edged calligraphic scripts.
I have some old shadow nibs from late model Osmiroid pens. Kaweco currently makes #5 Bock shadow nibs, but I am not aware of any others producing them commercially. Last year, at the San Francisco Pen Show, one of the Bay Area "Friends of Calligraphy" calligraphers had a Pilot Parallel that had been ground to a shadow nib by a friend. Just recently, Salman Khattak (smk on FPN) has offered customized Pilot Parallel shadow nibs for sale, and I bought one made from the 3.8mm model. Here are a couple photos. (No affiliation, blah, blah, blah)