Herbaria & Collections
Types - Search the Smithsonian (US) collection of nearly 2000 melastome type specimens.
Collections - Search the entire collection at the New York Botanical Garden (NY).
Types - Type specimens at the New York Botanical Garden (NY).
Types - Type and general specimens at the Harvard University Herbaria (HUH).
Types - Type specimens at the University of Florida herbarium (FLAS).
Paris- Search the Museum National d'Histoire Naturelle (P).
Collections - A directory of genera, species, and collection label data for Melastomataceae in the collection at the Instituto Nacional de Biodiversidad (INBio), Costa Rica.

Floristics
Flora Mesoamericana - Dig through layers of information on Central American plants.
Flora of the Cerrado, Sao Paolo, Brazil - A search of this database from the Base de Dados Tropical yields species along with nice photos.
Central French Guiana - A collections database of Melastomataceae from NYBG.
Acre, Brazil - This collections database is part of a NYBG floristics and economic botany project.

DNA
NCBI - Search GenBank for DNA sequences of melastomes.
IPCN - Index to Plant Chromosome Numbers.

Endangered species
Rare plants - Search the World Conservation Monitoring Center (part of the IUCN) for threatened species.
Rare trees - Similar to the previous link, this one provides data only on tree species.

Miscellaneous
Images - A collection of images of melastomes from Tropicos; many are type specimens.