<p>What are good libraries for C with datastructures like vectors, deques, stacks, hashmaps, treemaps, sets, etc.? Plain C, please, and platform-independent.</p>

As always, Google is your friend:
http://nixbit.com/cat/programming/libraries/c-generic-library/
specifically:
http://nixbit.com/cat/programming/libraries/generic-data-structures-library/