I recently discovered Guitar Pro 5 tablature software.  I had been using PowerTabs for quite some time but was growing frustrated with some of its limitations and the fact that there haven't been any updates in a long time nor any clear indication if there will be in the future. 

If you're not familiar with the idea, Guitar Pro, PowerTabs and other applications enable guitar players to easily compose or play music written in guitar tablature notation, also known as tab format.  As the old joke goes, the easiest way to get a guitar player to stop playing is to put sheet music in front of him.  Tablature is a more accessible way of writing out musical pieces for the guitar since the visual representation matches precisely to the frets along the 6 strings of the guitar.  If you can find, say the 5th fret on the 6th string, you can read music from guitar tabs.  Tablature has actually been around hundreds of years.  Although it is sometimes looked down upon in formal musical instruction, magazines such as GuitarWorld and books routinely publish songs in tab format for budding musicians to learn. 

Thanks in part to the Internet, there's been tremendous growth in the number of amateur produced tab files.  Typically these have been written in ASCII text format like the following (starting from the 4th bar):

In fact, I'm guessing that Lutists tabbed Louie Louie back in the 1400's -- though they could never quite decipher the lyrics.  Although tab files are helpful even in text format, these notations typically don't tell you much about the notes' duration; you can guess at it if you know the time signature since each bar has 4 beats in 4/4 time, but you can't hear a text file.   That's where Guitar Pro comes in handy.

Guitar Pro displays the tab notation below the standard staff notation so even if you can't really read music you can at least learn what symbols represent different note durations, like half notes, quarter notes, etc.  You can also view the fretboard or piano keyboard at any time, which makes it easy to identify the notes being played and visualize the patterns.  For example, as the fifth bar of the solo is played it will show you all the notes of that measure and the current note (G) in red.  Louie, Louie is kind of a crazy solo, but in other cases it makes it easy to recognize, say, a Pentatonic scale, a Blues scale, some weird Dorian scale and so on.

More importantly, Guitar Pro can play the music through your PC speakers so that you hear the song in real time along with any additional instruments that have been added, such as bass, percussion, etc.  While the quality of the rendition varies greatly depending on the effort that has gone into it, the guitar, bass and percussion tracks can be played using the Realistic Sound Engine, which sounds far better than the average MIDI file.  That enables you to exploit other features of Guitar Pro such as muting the part you are trying to learn.  GuitarPro also makes it easy to practice loops, slow down the tempo, etc.  You can easily import files in MIDI, ASCII text or PowerTab format.  And you can export the file as an audio WAV file and print the notation with lyrics and chords

As you can tell, I am quite impressed with Guitar Pro.  It seems I'm not the only one.  There are dozens of sites with quite literally hundreds of thousands of tab files in Guitar Pro format.  If you're trying to find a low-cost tool to help you learn or practice specific songs, or if you want to compose your own songs, Guitar Pro is worth looking into.  The trial version is good for 15 days and lets you check out most, if not all, of it's extensive set of features.

Update:
I didn't realize Guitar Pro 5 had the ability to display the fretboard until someone pointed that out to me last week at National Guitar Workshop in Chicago. 

• GuitarPro: Main, Trial Version, Documentation
• Tab Resources: TabScout, FireTabs, GProTabs.net