Opiates, while technically depressants, really belong in their own category.
Here's a list of all the classes of recreational drugs, with some representatives and what they do to neuro-receptors. I can't think of any other classes I could have forgotten, though whether or not entactogens really exist seperately from psychedelics.
FrostyMcFailure is somewhat accurate. This was just discussed in ADD, so my response may sound a little copied (I forget who wrote it, but it was a very good description). MDMA, according to our current understanding, binds to the same place SSRIs do, but instead of inhibiting reuptake, it causes the pump to work in reverse, flooding the synapse with serotonin.
It's also a 5HT2a agonist, and a Dopamine reuptake inhibitor, so it does supply it's own goodness, as well. It'd still be a wonderful substance even if it didn't flood your brain with serotonin.
I sometimes wonder if MDMA might be an atypical entactogen. If we just happened to find a substance that's rather unique in it's effect profile, and that there's another group of substances that'd do the same thing- but as a group would be more uniform.
Depressants: Benzodiazepines, Glutethimide, Secobarbital, Methaqualone, etc- GABA agonists
Stimulants: Cocaine, MDPV, Amphetamine, Meth, Desoxypipradol- Dopamine Reuptake Inhibitors
Psychedelics: AMT, DMT, LSD, 2C-B, Mescaline, 2,5 DMA, etc- 5HT2a agonists
Entactogens: MDMA, Methylone, possibly butylone and ethylone. Not very many here. Serotonin releasers, some 5HT2a agonism.
Opiodergics: Heroin, Morphine, Fentanyl, long freaking list. mu-opoid receptor agonists.
Dissociatives: PCP, PCPy, PCE, Ketamine, N2Oanother long freaking list of oddballs. Salvia is sometimes lopped in here. NMDA antagonists, Sigma receptor agonists (possibly- not always), Kappa receptor agonists (in some cases, namely Salvia, it may play an important role in other atypical dissociatives).