<p>Does anybody have an implementation of <a href="http://en.wikipedia.org/wiki/Cuckoo_hashing" rel="noreferrer">Cuckoo hashing</a> in C? If there was an Open Source, non GPL version it would be perfect!</p>

<p>Since Adam mentioned it in his comment, anyone knows why it is not much used? Is it just a matter of implementation or the good theoretical properties do not materialize in practice?</p>

Cuckoo hashing is relatively unused outside of academia (aside from hardware caches, which sometimes borrow ideas from, but don't really implement fully).  It requires a very sparse hash table to get good time on insertions - you really need to have 51% of your table empty for good performance.  So it is either fast and takes a lot of space, or slow and uses space efficiently - never both.  Other algorithms are both time and space efficient, although they are worse than cuckoo when only time or space is taken into account.
Here is a
code generator for cuckoo hash tables
.  Check the license of the generator to verify that the output is non GPL.  It should be, but check anyway.
-Adam