I've been playing with the TeamCity API lately and I've had many ideas for plugins and other generic extensions not strictly related to plugin development. As they started to grow I felt the need to publicize them and what I've done so far so that someone can eventually jump on the train.
C# runner plugin - represents a build runner which is able to run C# code on the agent, much like LINQPad but on TeamCity
Dependency graph plugin - a custom tab which shows dependencies between build types in a graph
TFS issue tracker integration - integrates TFS into TeamCity
Pivot viewer for TeamCity - based on the new pivot viewer control for Silverlight, a proof on concept for displaying TeamCity projects/buildtypes/builds based on the example available here
- Google project hosting issue tracker integration with TeamCity - just an idea right now, there is a Java API and would be useful for OS projects using Google hosting
- TC build runner to run other builds - an idea as well, would provide a means to trigger other builds, useful when you want to batch a group of builds, for example. This funtionality is already available via HTTP calls to TC