There's too many stimlants to list. Define "low melting", ballpark figure?
The one that springs to mind would be methamohetamine HCl, it melts fairly quickly at 170-175C, incredibly water soluble usually present as "shards" or small needlelike crystals.
Caffeine could be needlelike crystals but it sublimes rather than melts.
Cocaine is not likely to be needlelike crystals and is easily diagnosed by putting a speck on your tongue: cocaine is an anesthetic, basically nothing else is.