I am looking for a lean and well constructed open source implementation of a B-tree library written in C. It needs to be under a non-GPL license so that it can be used in a commercial application. Ideally, this library supports the B-tree index to be stored/manipulated as a disk file so that large trees can be built using a configurable (ie: minimal) RAM footprint.
Note: Since there seemed to be some confusion, a Binary Tree and a B-Tree are not the same thing.
Check out QDBM: http://fallabs.com/qdbm/. It's LGPL (can be used in commercial app), implements a disk backed hash and/or B+ tree with arbitrary key/value pairs, and builds on a variety of platforms.