Plug-ins have become an important part of today's integrated development environments (IDEs). They are useful for extending the functionality of these environments and customizing them for different types of projects. In this paper, we discuss some features that should be provided by IDEs to support the development of a specific kind of plug-ins - plug-ins that support program analysis and software testing techniques. To guide the discussion, we leverage our experience in building a plug-in for two different platforms and generalize from that experience.