Funds provides portable, purely functional data structures in Common Lisp. Funds was developed as a Google Summer of Code Project. Read more about my motivation for developing funds, and a description of the effectiveness of using tree-backed data structures in my Google Summer of Code application, funds.pdf. Funds is open source software released under the Apache License.
The current release of Funds is here: funds.tar.gz.
You can browse the Subversion repository or download the current development tree via anonymous SVN, as described here