When learning a new song, it’s really helpful to be able to play along with the original track, and slow it down while preserving the original pitch.
Without special processing, if you just slow down audio playback, it’ll get lower in pitch (remember slowing down a spinning record and hearing it get low and slow?). And if you speed it up, it’ll raise the pitch (chipmunks!)
I use the tape deck in Native Instruments Guitar Rig, which is great. It lets you slow down the audio, independently change the pitch if you want, set loop points, and speed up/slow down the track.
Being able to control the pitch independently of speed is great for playing songs that are not tuned to standard A440, without having to retune your guitar. Some examples I like to play are Weezer’s Say It Ain’t So (which is tuned down a half step), and Police Every Breath You Take (which is tuned down about 70 cents).
Of course Guitar Rig models an entire rack of gear and effects, along with a nice looper, so it’s a pretty amazing tool for playing and learning.
I also found (but haven’t tried) some simpler standalone slow-downer tools like:
Guitar & Drum Trainer
The Amazing Slow Downer For Windows
Free online Shockwave app Slow Notes (which looks promising but didn’t work for me).