There's a lot of software that's guitar related, but most of them are expensive. All of the following list is composed of only free software that you can download.

The best (in my opinion) guitar tab editor.

AWS Guitar Tuner 2.01
Guitar tuner that helps you tune to standard, open G, open D, open A, open E, E minor, drop D, double drop D, half-step down, and that one funky tuning used in 'Going to California: DADGAD.

RiffWorks T4
Excellent drum machine.

Another great tab editor.