Free Software for The Logic Book
If you can handle .zip files, you can download the software for The Logic Book for free from http://www.ucc.uconn.edu/~wwwphil/software.html. There are two pieces of software there, both written by Austen Clark at Storrs, TWOOTIE and BERTIE3. The first deals with truth-trees and the second deals with natural deduction (i.e. SD, SD+, PD and PD+). Each program functions both as a mentor, giving you feedback as you try your hand at stored problems, and a proof editor, identifying the errors in any proof you type in.
I use BERTIE3 with my students, and have prepared a guided tour. For those who do not have WinZip, here are some self-extracting zip files.