Stopping short of actually playing with other musicians, they are the best way to help evolve your playing. Much like learning a language by moving to the country that speaks it!
I recently came across the great guitarplayback.com website from David Wallimann and Dan Davies which provides a great range of high quality backing tracks in a variety of styles and levels to suit beginner to advanced players.
The Internet is covered with websites designed to provide you with backing tracks to your favourite songs, so that you can jam to any song by any band that you can think of. We didn’t want to be another one of those websites. Guitar Playback was created to help guitar players develop their musical ability by offering very high quality, custom-made backing tracks at an affordable price. Each track is written to help you master a specific musical concept suggested in its description.
Beginner backing tracks are all written using a single scale. This collection of tracks is perfect for guitarists who want to develop their musical vocabulary by learning new scales and patterns.
Intermediate backing tracks are written in two different keys. These backing tracks require the user to have a good knowledge of the keys involved, and whilst they are a bit more challenging than the beginner tracks, the modulations offer a very interesting educational value.
Advanced backing tracks contain at least three different keys. This section requires the user to have a good understanding of music theory as at least two modulations are involved. The content found here will force you to think about what you are playing, which is key to good improvisational skills.