If you are learning to play diatonic harmonica and the blues in particular you will eventually need to master bending notes and over blowing notes. Both difficult to explain and difficult to practice.
Harpsoft.com have developed a nice piece of Java software to help.
Just fire up the application, choose the key of harmonica you are using and the tuning, at the moment you can choose between Richter, Country, Lee Oskar Melody maker, natural Minor and chromatic ten hole.
Now, with a microphone on you computer you get a visual indication of the note you are playing. You can see the note bending down and also get a precise indication of how far you have bent the note.
This is a really invaluable learning tool giving an accurate representation of your performance. It is also linked to a database of song tabs so you can use it to practice songs by following the notes as they are displayed on the screen.
This is not freeware however and to use the software after the initial demo period will cost $25 for a twelve month licence, i.e. about half the price you would pay for an harmonica for twelve months use of this excellent tool.