The Debian Bug Tracking System is unusual in that all input and manipulation of issues is done via email: each issue gets its own dedicated email address. The DBTS scales pretty well: http://bugs.debian.org/ has 277,741 issues, for example.
Since interaction is done via regular mail clients, an environment
that is familiar and easily accessible to most people, the DBTS is good
for handling high volumes of incoming reports that need quick
classification and response. There are disadvantages too, of course.
Developers must invest the time needed to learn the email command
system, and users must write their bug reports without a web-form to
guide them in choosing what information to write. There are tools
available to help users send better bug reports, such as the
command-line reportbug program or
the debbugs-el package for Emacs. But most people
won't use these tools, they'll just write email manually, and they may
or may not follow the bug reporting guidelines posted by your
project.
The DBTS has a read-only web interface, for viewing and querying issues.