You can use -links if your filesystem is POSIX compliant (ie, a directory has a link for each subdirectory in it, a link from its parent and a link to self, thus a count of 2 link if it has no subdirectories).
The following command should do what you want:
find dir -type d -links 2
However, it does not seems to work on Mac OS X (as @Piotr mentionned). Here is another version that is slower, but does work on Mac OS X. It is based on his version, with correction to handle whitespace in directory names:
find . -type d -exec sh -c '(ls -p "{}"|grep />/dev/null)||echo "{}"' ;
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