Here's a list of everything that we're aware of that works with FogBugz from outside FogBugz. Note that there are plugins in the FogBugz plugin gallery that integrate FogBugz with outside systems, and that we have built-in integration with most every source code control system. Disclaimer: Third party applications are not tested by Fog Creek and there is no warranty express or implied. Another list is maintained at http://www.fogcreek.com/fogbugz/extras.html.
Command Line Client: The FogBugz command line client fbcli, which uses the XML API to talk to FogBugz via the command line on windows, mac, and linux.
FogBugz Screenshot Tool: Built into FogBugz. You can attach to a case, an email, or just save the image to disk.
SnagIt Mashup: FogBugz and SnagIt joined together so you can add arrows, torn edges, and other cool SnagIt features
BugShooting Screenshot Tool by Alexej Hirsch - an advanced screenshot tool with detailed editing.
Trac Importer : Import your Trac bugs and wikis into FogBugz
Confluence: Import your Confluence wiki into FogBugz
Dragnet: Import Dragnet database into FogBugz by Ken Bonnin, Jr.
Mantibugz: Import your Mantis database into Fogbugz by Martin Zardecki
DBxtra for FogBugz by DBxtra: Report Software for Easy Use by Anyone.
Tableau Software: Fast Analytics for FogBugz
Filter List PHP Script: Generate a single-page list of case views for all cases in a given filter by Ted Stresen-Reuter
Test Case Management
TimeSprite by Black Hill Software: Exchanges time tracking data with FogBugz. View FogBugz time tracking data. Track time offline and submit to FogBugz.
WorkingOn for FogBugz system tray tool by Daniel Schaffer.
Timepost: Available for both Windows and Mac. Integrates with FogBugz.
CaseTracker: An open-source Windows tool to set working on case, resolve the current case and create a new case.
Plugins for Third Party Applications
Bugclipse - An Eclipse plugin that integrates with FogBugz, featuring a rich editor, focused UI, offline support, attaching code contexts to cases, automatic time tracking and export of cases in various formats (including HTML, PDF and XLS).
Foglyn - An Eclipse plugin that works with the Mylyn task-focused interface. It allows you work with your FogBugz cases from within Mylyn.
MT Fogger - a Movable Type interface to FogBugz. Exposes a set of template tags that make it possible to query and display information stored in FogBugz within Movable Type templates.
SourceForge.net: EclipseBugz - An open source Eclipse IDE plugin.
Scripts & Example Source
Python Sample API Code - Python API Class Libraries - Work with FogBugz easily in Python
FogBugz API Explorer by Paul Mignard: Execute FogBugz XML API commands in this small cross-platform AIR app and browse the resulting data as XML or a pretty tree view.
APITester is a .NET app that runs through most of the XML API and can be used to debug and learn from.
C# (CSharp) Wrapper Class for FogBugz API - EJ Alexandra has written a C# class that makes using the XML API in .NET painless.
RBScoutSubmit started by Ian Jones: Open source ScoutSubmit implementation for REALbasic applications.