Flyspray is a web-based bug tracking tool. See http://flyspray.rocks.cc/. Prettier than Mantis; much easier to install and configure than Bugzilla (but less capable).

It's implemented in PHP. It uses a database backend via ADODB (MySQL, PostgreSQL). Flyspray is licensed under LGPL 2.1. Other co-requisite packages are licensed under various free (as in freedom) open source licenses. Current (1-May-2005) release is 0.97.

It has features for:
* users, permissions for users by activity, by project.
* projects have users and project permisssions (OR'd with user's base permissions), description, visibility, others.
* reports have attributes: project, title, description, reported by, comments, categories (think component), status, comments, attachments, reported against some level, ... 
* close codes, affected OS, ... a number of other attributes of reports are configurable.

Stable in use on Win2K with XAMPP. Target environment is Linux.
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Just installed it here, as part of package selection. My initial impression is that it is more than good enough for my needs. I'm a one-man-shop developing web applications. In truth, Flyspray is a bit rich for my needs, but I'll just not use some of the features. The feature set looks rich enough to support many projects with at least a hundred users. More?