Economic models
Reader pays through subscriptions/licenses (eg most established printed journals)
Author pays through page charges/handling charges; free to readers (eg BioMedCentral (BMC))
Institute/Society/University pays, through increasing charges for other services, advertising, subscription savings; free to readers (eg BMJ, online-only Bioline journals... )
Differential charges to different readers (eg BMC, HINARI programme, PERI programme…)
Government or other grant (eg SciELO, Brazil); free to readers
Free through institutional e-print archiving (arXive, increasing numbers of publications adopting this policy)