Xah Lee, 1999, 2005
Tree::Matica is a Perl module of tree-processing functions. Download the module here: Matica.pm (yes, just a single file.)
I'm starting a project that will write each of the tree functions in Perl, Python, Java. Please see http://xahlee.org/tree/tree.html
Stefan Holdermans has written a Haskell version. See http://www.cs.uu.nl/~stefan/blog/00002.html
Disclaimer: Mathematica is a trademark and product of Wolfram Research Incorporated. The Perl module Matica.pm is not affiliated by Wolfram Research Inc.