Redmine

Adds external watchers to Redmine issues: any e-mail address, say a customer's or a contractor's, that receives the same notifications as Redmine's watchers without a Redmine account and without access to Redmine. Addresses are managed in the issue sidebar below the watchers, follow Redmine's e-mail notification settings, never receive private notes, and every mail carries an unsubscribe link that works without login. Two permissions (view, manage) in a project module. No settings, no extra gems. Commercial plugin, one-time purchase per Redmine instance, 12 months of updates, full source code (GPLv2).