I like the Green Rhino, although it's a little bloated and gainy to put on an already gainy amp. The Green Rhino does a great job on clean amps like Fenders and Matchless.
On a gainy amp, I really like the cheap MXR Classic OD/GT-OD. They add just a little bit of shimmer and gain with the gain set to around 9:00. I like the MXR on Marshalls that already have some gain.
Check out some of the Joyo pedals too, like the Sweet Baby.
If you want a nice clean amp to put next to your Dark Terror rig, just pick up something like a Blues Jr.