I have requests from our customers about the ability to install our (commercial) plugins on JIRA Hosted (http://www.atlassian.com/software/jira/hosted/). Is it possible for them to do if our plugins are not on the list of JIRA Hosted add-ons (Greenhopper, Gliffy, Balsamiq)? What should be done to enable third-party plug-ins?
Just a quick heads up for people finding this via a search, this answer is in regards to JIRA Hosted, a service which is no longer available. The new hosted option for JIRA (and other Atlassian applications) is OnDemand. With JIRA OnDemand, plugins are handled differently...please refer to our plugin policy for further details.
Keep in mind that if your commercial plugins are licensed per-user, your customer will actually have one more user than JIRA allows, i.e. 25-user license hosted JIRA will have a limit of 26 users. We have to account for it ourselves by generating a license one user more than purchased.
JIRA Hosted instances (not JIRA Studio) are allowed for plugins installation on ad-hoc basis. User accounts with system administrator privilege should be able to conduct the installation via the Universal Plugin Manager.