Fixing jinja2 and pycrypto (and probably others) on Snow Leopard
I use the python.org installs for my system Python. However, these installs assume a 10.4 SDK on the system, which Snow Leopard does not install by default - it's an optional install in the developer tools installer. This causes the C bits to fail with a dramatic flourish of errors. Luckily, there are two easy ways to fix it. The first way, and probably the better one, is to just install the 10.4 SDK. If you're too lazy to hunt for your SL CD (though it's probably early enough that it's still sitting on your desk somewhere) you can modify /Library/Frameworks/Python.framework/Versions/Current/lib/python2.6/config/Makefile and replace all instances of 10.4u with 10.6.
There is no way that I figured this out on my own - the wonderful folks in #pocoo on irc.freenode.net helped me out (special thanks to mq and mitsuhiko). Also thanks to this reddit post which contained a few pointers.
(note that this is a Python issue, not a Jinja2/PyCrypto/whatever issue)