Any recommendations for a startup graduating from emails and Skype chats to a ticket management system for support and keeping track of development?

Desired features:

- Relatively lightweight, this is the "bigger than a couple of people inside each other's heads, smaller than a giant organisation with management tiers" scale. Aiming to reduce busywork, not increase it!

- Integrates with email well for support.

- Hosted, or can be self-hosted on Linux.

- Open source a plus but not a fixed requirement.

Considering but have not yet evaluated:

- Request Tracker http://bestpractical.com/rt/

- Eventum http://dev.mysql.com/downloads/other/eventum/

- Zendesk http://www.zendesk.com/

What else would you suggest looking at (or avoiding)?