I've already got a modded Ibanez Tube Screamer but I like to kick it in the butt sometimes by adjusting the output of my Barb EQ or my Barber Tone Press.  I want to save myself the trouble of bending over and changing settings by adding another boost pedal in the signal chain.   
 
What do you have doing the same sort of thing?

I have a Marshall BB-2 Bluesbreaker II, which has both clean boost and "blues" modes. The blues mode does a nice job of adding just a bit of dirt without messing things up.

A Tube Screamer and a RAT.

I have one of these:
http://www.rockshop.co.nz/shop/crowther-hot-cake-overdrive-pedal.html
 
It is a bit of a legendary piece of kit. The guy who invented it played in Split Enz. Not sure if you can source them over there but if you get a chance you should demo one.

I use a Maxon OD808 for a clean boost.
 

MXR M104 Distortion

I love it.

I love my Barber Direct Drive. It's plays nicely with my TS.
 
Rocky

Siegmund Double Drive, Lovetone Brown Source and  Jacques fuse blower - mainly  use the Siegmund and Lovetone as I like the tones I can get from them - the Fuse blower can be a bit more extreme
Nigel
 

I love my Mesa Boogie "Tone Burst", it's clean and powerful. Has true tube tone. And it's as clean or slight breakup if you want. It also has great bass and treble controls, along with a level and gain controls. I think it even has some slight compression going on, but that may be in my mind!!

I have it set to clean with a slight boost and keep it on all the time. I love the tone and how it affects the attack of my playing. I have it going into my other distortion boxes.

I may get rid of some of my other distortion boxes, but not this one. It's a keeper for sure.

Mooer Green Mile.  retail $75 Can. 
I sat in a store and tried a whole bunch of overdrive pedals. The Mooer sounds like my Fender Princeton when I turn it up. I play mostly clean but when I want overdrive I want it to sound like I just turned the amp past 7 but without the volume boost. This does that perfectly.
Plus see the little toggle?  You can use that to just get a volume boost without adding any more overdrive too. Two pedals in one, very versatile.  
It's obviously trying to clone a Tube Screamer. I have a brand new tube screamer and the Mooer blows it away for Bluesy overdrive, way smoother. 
 
http://www.mooeraudio.co.uk/c/pedals
 
 

 
http://www.tomleemusic.ca/index.php/guitars/guitar-bass-effects/163784.html