3 free and easily available tools for usability testing

Problem Steps Recorder
Windows 7 comes pre-installed with "Problem Steps Recorder".
Go to "Start", search for "Problem Steps Recorder". Under settings, remember to set the number of steps as 100.

TeamViewer
Allow remote access.

InvisionApp
More for designers, but you can upload a screenshot and let your clients comment on the mockup.

Better than screenshots or emails, saves a lot of frustration.