Maxon ROD881 - Valve Overdrive/Distortion.
The overdrive setting is tubescreamer sounds and the distortion setting is for heavier tones.
There is a footswitchable boost to both these settings.
I have used it with Line 6 and Behringer solid states, which it made them sound like pure valve and a fender blues deville - which was a fantastic sound, great cleans then great overdrive ( and a boost).
My new amp has all the overdrive I need - I might pick something up for a lead boost sometime. They retail for £185. I'm asking for £95
Here's a bit of info. and the link to harmony central.