Lightroom’s crop tool allows you to constrain the aspect ratio to a proportion of your choice, e.g. to 4:3, defaulting to the same aspect ratio as the original. The last 5 or so custom crop aspect ratios are saved, but a minor annoyance is you are unable to clear the list.

Python on the Mac and SQLite to the rescue: this simple script  lraspect.zip will reset them. If you use a non-default name for your Lightroom catalog, you will need to edit it. To run it, quit Lightroom and run the script. It will back up your catalog for you just in case.

Needless to say, I cannot be held liable if this script corrupts your catalog or eats your dog (who ate your homework), use at your own risk.

#!/usr/bin/python
import sys, os, sqlite3

# edit this to point to your LR3 catalog if you do not use the default location
lrcat = os.path.expanduser('~/Pictures/Lightroom/Lightroom 3 Catalog.lrcat')

os.system('cp -i "%s" "%s.bak"' % (lrcat, lrcat))
db = sqlite3.connect(lrcat)
c = db.cursor()
c.execute("""select value from Adobe_variablesTable
where name='Adobe_customCropAspects'""")
crops = c.fetchone()[0]
print 'aspect ratios:', crops
c.execute("""update Adobe_variablesTable
set value='{}'
where name='Adobe_customCropAspects'""")
db.commit()
print 'Custom crop aspect ratios reset successfully'