There won't be any further plugins available in OnDemand, we're moving to the new development platform Atlassian Connect (https://developer.atlassian.com/display/AC/Atlassian+Connect) that allows developers to create Add-Ons for OnDemand that you will be able install. Any existing plugins that you're interested in will have to be ported to the new platform (and you might want to contact the vendors to encourage them to do this) and once Atlassian Connect is out of beta you'll be able to add these to your instance via Marketplace.
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