I've been useing this for the last couple of days and it seems very useful.

http://bestdrumtrainer.com/st/

Here is the description on the webpage:

Speed trainer is a metronome-like tool that allows you to train stability and speed of your tempo. It differs from typical metronome - speed trainer can automaticaly increase and decrease BPM.

Is this for drummers only? NO! If you play guitar, piano, violin, flute, trumpet or any other instrumment you can use it. I hope it will be helpful.

Happy Guitaring,

Jim