Give to a friend or family member. Donate. If you participate in any casual group rides like the monthly Critical Mass in some cities you'll find some bareheaded folks. Maybe one of them will use it.

I'm keeping my crashed helmet for an art project. Partly a reminder of how idiotic some drivers can be, partly to remind me to always wear a helmet, partly because I enjoy kitsch art. I'm just looking for a properly damaged mannequin head to put it on. Maybe turn the whole thing into a lamp or punch bowl.