Of all the defensive devices recommended, both fair and foolish, in this thread, only the walking stick is both absolutely and undeniably legal in every jurisdiction, and moderately effective (although not a good stand-off weapon.)
A few people have mentioned 'canes' (commercial ones are usually too flimsy for weapon use) or 'blackthorn' sticks: I haven't seen a real blackthorn since my grandfather's day. Since I have become afflicted with arthritis, I have taken to fabricating my own sticks...a root-ball and 36 to 39 inches of 1-1/2" pin cherry, hop hornbeam, apple, maple or oak makes a good stout walking aid which is also a fearsome weapon when properly deployed. And even the Transportation Safety people will let you take one aboard an airplane, provided there are no metal parts involved. I wouldn't waste money on OC, or any of the other, mostly illegal impact weapons suggested by earlier posters in this thread, and will stick with my stick, thanks.