Ask HN: Share your C wisdom (a.k.a C programming redux 2010)

I learnt programming with C when I was 16, but today I mostly program in Python. C is a compact language. Learning the fundamentals of the language itself does not take a lot of time. But, what are the things/libraries/practices a casual C programmer like me should know to be productive and "safe".

For example, I recently came across bstrlib (better string library) while reading mongrel2 source code. Are there any other libraries that are "a no-brainer", "use it all the time"?