Clean Boost – For those guitarists who like to get natural distortion from their amp but want a db boost and sustain for their solos, clean boost is the way to go. This is an old school way of getting sustain and tone from your amp and can be done with a variety of devices, not just the ones labeled as boosters. For example, when I was touring with my rock band in the early 80s, I couldn't get enough gain out of my Marshall amp to get the sustain I wanted so I tried a variety of methods. I ended up using a graphic EQ with the mids boosted for my solos. Mike Schenker uses a Wah-Wah pedal turned on and left alone to get a similar effect. There are some pedals made specifically for clean boost but an overdrive or distortion pedal can be made to do something similar.

How to use clean boost – Your amp has to be in some stage of breaking up to make a clean boost work for you. Some of the pedals made specifically for this will sound pretty lame through a crystal clean amp. Basically when using an overdrive or some other kind of clipping device to get the sustain you need, you set the volume up and gain down on the pedal. When I first started using an Ibanez tube Screamer, I set the volume on ten and the gain on about one or two. This would help me get above my band for my solos and add sustain and warmth. Even though amps are made with much more gain these days, I still tend to turn my gain to three or four on my Marshall to get a nice crunchy sound, and rely on a clean boost for my solos, making the amp and pedal work together. Some are very transparent, meaning that they don’t have to shade your tone at all and can be made to boost only volume and will usually allow you to roll on gain as well (like Xotic's RC Booster). Others have their own tonal characteristics (like the Ibanez Tube Screamer).

First get a nice warm crunchy sound on your amp. Turn your boost or overdrive pedal on and tweak the volume, tone and gain so that the tone you get is as close to the same sound you get when it isn’t turned on. With the Xotic RC Booster, I can make it sound exactly the same on or off, that is how transparent to pedal is. Now start adding volume and gain to the pedal to get more sustain and volume. Use the natural crunchy sound for you rhythm and turn on the pedal to get up above your band for your solos.
On to the history makers, old and new:

Butler Audio (Chandler, Tube Works) Tube Driver – More of an overdrive unit than a true clean boost, the Tube Driver actually has a single 12AX7 pre-amp tube on board to create just enough warmth and sustain for solos. The most notable users would be Eric Johnson, Billy Gibbons, David Gilmore and Joe Satriani (for a while). Designed by Brent Butler and first manufactured by his company Butronics in 1978 or 1979 later changed to Audio Matrix until the company folded in 1982. Paul Chandler later did the distribution, thus the most commonly referred to name, the Chandler Tube Driver. This also led to litigation on who actually owns the design. It was also issued under the name Tube Works Tube Driver for a while. Brent Butler still manufactures a limited amount (6 units a week) under his current company name, Butler Audio. It will also set you back about three hundred dollars. Info: http://www.tubedriver.com/tubedriver.php

Ibanez Tube Screamer – The Tube Screamer has been used by countless guitarists since its debut in the late 70s. Most notable is Stevie Ray Vaughn who used one throughout his career.  The first model was the TS-808 (which Stevie used) followed by the TS9 in the early 80s. These proved to very popular as well and are the ones mostly seen in pedal boards. The newer models starting in the late 80s and 90s such as the STL and TS10 proved unpopular. Generally Tube Screamers are used to boost an already overdriven amp and set with the volume up full and the gain set back to about one or more depending on how much the amp breaks up naturally.

Xotic EP Booster – Both Jimi Page and Eddie Van Halen have one thing in common; both supposedly at one time or another used the Echoplex EP-3 to boost their Marshall amps. Now Xotic has designed a booster mimicking the boosting characteristics of the classic EP-3. It offers 20 db of boost and you can change the EQ and boost frequency settings inside. An added benefit is the size, it is relatively small and won’t take up too much space on your pedal board. More here: http://xotic.us/effects/ep_booster/

Xotic RC Booster – Popular for their natural and warm sounding tones, Xotic makes a series of overdrive boxes that many session players use, Steve Lukather, Scott Henderson and Greg Howe to name a few. Like Xotic's EP Booster, The RC Booster offers a 20 db boost. It is very transparent and won't change your tone at all. But if you want to tweak your tone, you can with its separate treble and bass knobs. Some players use it in the front of their effect chain, leaving it on all the time for a dbl boost and tone warmer. As it is designed for boost, it is does not offer over the top distortion. It is designed to work in unison with your amp's natural distortion. It is true bypass, meaning when switched off, it won’t affect your signal chain. Info: http://www.xotic.us/effects/rc_booster/