FerryLeaks is an editor for the table algebra, developed for debugging query plans for Ferry and Pathfinder. It supports SQL-generation for either the whole plan or sub-plans, features 'SQL-Listeners' that can be attached to any node and provides editing in a graphical manner as well as through direct XML-manipulation.
Take a look at the example installation.
Moreover, FerryLeaks ships with a simple XML descriptor language that is used to define and validate nodes and plans. It is highly flexible and can be used as an editor/evaluator for almost any kind of algebra that can be expressed in a tree-like structure. A testing example for FerryLeaks, loaded with descriptor files for a simple algebra supporting literals, 'plus', 'minus', 'mult' and 'div', can be found here.
The code can be found on github.