RSE has done the maths on why *33 is a magic string hashing function...
this from his "str" library in str_hash.c:
/*
** Str - String Library
** Copyright (c) 1999-2000 Ralf S. Engelschall
**
** This file is part of Str, a string handling and manipulation
** library which can be found at http://www.engelschall.com/sw/str/.
**
** Permission to use, copy, modify, and distribute this software for
** any purpose with or without fee is hereby granted, provided that
** the above copyright notice and this permission notice appear in all
** copies.
**
** THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESSED OR IMPLIED
** WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
** MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
** IN NO EVENT SHALL THE AUTHORS AND COPYRIGHT HOLDERS AND THEIR
** CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
** SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
** LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
** USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
** ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
** OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
** OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
** SUCH DAMAGE.
**
** str_hash.c: hashing functions
*/
#include "str_p.h"
/*
* DJBX33A (Daniel J. Bernstein, Times 33 with Addition)
*
* This is Daniel J. Bernstein's popular `times 33' hash function as
* posted by him years ago on comp.lang.c. It basically uses a function
* like ``hash(i) = hash(i-1) * 33 + string[i]''. This is one of the
* best hashing functions for strings. Because it is both computed very
* fast and distributes very well.
*
* The magic of the number 33, i.e. why it works better than many other
* constants, prime or not, has never been adequately explained by
* anyone. So I try an own RSE-explanation: if one experimentally tests
* all multipliers between 1 and 256 (as I did it) one detects that
* even numbers are not useable at all. The remaining 128 odd numbers
* (except for the number 1) work more or less all equally well. They
* all distribute in an acceptable way and this way fill a hash table
* with an average percent of approx. 86%.
*
* If one compares the Chi/2 values resulting of the various
* multipliers, the 33 not even has the best value. But the 33 and a
* few other equally good values like 17, 31, 63, 127 and 129 have
* nevertheless a great advantage over the remaining values in the large
* set of possible multipliers: their multiply operation can be replaced
* by a faster operation based on just one bit-wise shift plus either a
* single addition or subtraction operation. And because a hash function
* has to both distribute good and has to be very fast to compute, those
* few values should be preferred and seems to be also the reason why
* Daniel J. Bernstein also preferred it.
*/
...