The clear jelly you find in pork pies is a typical natural meat gelatin, although gelatin can be derived from a variety of sources. This includes isinglass, from the swim bladders of fish (and used to clarify beers and wines), and hartshorn, made from the antlers of deer.
Gelatin is widely available in the form of sheets or leaves – which have to be soaked before use – or as an instant powder. Vegetarian alternatives include agar-agar, carrageenan and pectin.
Recipe that use gelatin:
Lingonberry and orange cheesecake with a gingersnap crust
Raspberry cheesecakes
Ricotta filo tart with figs, rum and toasted coconut