>What's TMA?
>Does anyone know?  Some sort of MDMA derivative?

Well in a vague sense it is related to MDMA, in as much as it
is a methoxylated amphetamine derrivative.

  MDMA:  / \      3,4,5 TMA:       
        O   O               H3C-O     O-CH3  
        \___/                    \___/
        / 0 \                  __/ 0 \
        \___/                 O  \___/
            \__/              |      \__/ 
               \              CH3       \
                NHCH3                    NH2

There are six possible analogues of TMA:

TMA: 3,4,5 trimethoxyamphetamine
Dose: 100-250 mg Duration: 6-8 h
Effects: Similar to mescaline but perhaps less pleasant. More active.
This is the amphetamine analogue of mescaline, and may be derrived
from elemicin, a fraction of nutmeg oil.

TMA-2: 2,4,5 trimethoxyamphetamine
Dose: 20-40 mg Duration: 8-12 h
Effects: Strongly psychedelic, somewhat similar to mescaline. Pleasant.
Derrived from assarone, a component of calamus oil.

TMA-3: 2,3,4 trimethoxyamphetamine
Not found to be active at 100mg

TMA-4: 2,3,5 trimethoxyamphetamine
Slight activity (~= 50ug LSD) found at 80mg ca. 6 h

TMA-5: 2,3,6 trimethoxyamphetamine
Mild stimulant and psychedelic effects found at 30mg (~75 ug LSD) 8-10 h

TMA-6: 2,4,6 trimethoxyamphetamine
Dose: 25-50 mg Duration: 12-16 h
Effect: Very pleasant stimulant and psychedelic

(Information taken from Pihkal by Alexander and Ann Shulgin)

If TMA is found on the illicit market, it is almost certainly TMA
or TMA-2 because of the ease of synthesis. I would be interested
to hear the experiences of anyone who has tried these materials,
or has come accross them.