Boosters and Equalizers

A booster will give you a boosted signal, i.e. more volume. All boosters does change the signal, some more than others. Some will even do it on purpose.

Many other effect circuits can be used as boosters, given that the original effect of the circuit is dialed down.

Links

Tillmans preamp - Super simple boost (that does not include a volume control by default).
EBC41 double-diode triode booster - Booster using sub mini tube.
Rump Roast - Can be built as a booster or OD.
LPB-1 - Linear power booster, classic boost.

Sites with mutliple effects

Run Off Grove - Fetzer Valve, May Queen, Mr. EQ, Omega, Tonemender.
Tonepad - AMZ Mosfet Booster, Brian May's Treble Booster, LPB1, MXR MicroAmp, Purple Peaker, Range Master, Real McTube, Red Ranger.