Ask HN: What's the best web-based issue tracking system?

I'm working for a medium-sized company building web apps both internally and for external customers across the globe.

We're currently using Atlassian's Jira for issue tracking, but we're not pleased and we're looking for alternatives, preferably something with the ease of use and scaled down experience similar to 37signals products.

The basic feature list we are looking for is:

  - Web based
  - Ability to manage issues by project
  - Ability to isolate users to specific projects
  - Ability to invite external users (customers) to the system

I'm turning to the HN community to help me discover some of the lesser known but still (hopefully) great products out there. Any recommendations?

Big thanks.