I've been eating it in mixed green salads, with radishes and/or cucumbers and/or goat cheese. (I wouldn't put too much of it in at once; it's supposed to be a laxative in large quantities.)

I've also put some into a tzatziki to eat with lamb burgers, and that was fantastic.

Joy of Cooking also recommends a chiffonade of sorrel in soups. I haven't tried that yet, but I bet it would be excellent in a ministrini, or as a garnish to a potato soup or a green pea soup.