Speedballing (heroin + cocaine IVed).
Shrooms + crack.
Heroin + temazepam + cannabis.
GHB (or if not available, alcohol) + crack/cocaine.
GHB + temazepam.
GHB + MDMA.
LSD + MDMA.
MDMA + heroin.
Most of these combos benefit from the addition of some nice, dank weed as well.
But the combo I take every single day is methadone + clonazepam + temazepam, plus weed to top it off. The first three I get legally (MMT + scripts for the benzos), the weed obviously not. It's marvelous. It's what keeps me off the hard drugs these days, knowing I get a legal cocktail of three great drugs that have a wonderful synergy between each other. Don't need anything else.