Ask HN: What’s the best open source helpdesk?

I’m on a sabbatical helping a nonprofit reign in their donor relations and volunteer email support.

Zendesk and the usual suspects are way out of their budget.

What are your recommendations for a user friendly open source helpdesk solution?

Bidirectional email support, SLAs, assignment, and prewritten responses are nice to haves.

Wikipedia has a useful comparison of issue-tracking systems, mostly IT focused though:

These might be closer to what you are looking for but I can't recommend them because I haven't tried them. GPLi has my vote for tech support.

You could also consider a CMS (Wordpress, Drupal, etc.) help-desk plugin