As the need for complex parallel simulation software grows, better strategies for efficient and effective software development become important. We advocate a toolkit --- or ``tinkertoy'' --- approach to parallel application development. By providing efficient implementations of basic services commonly needed by applications, toolkits allow application developers to benefit from others' research, compare algorithms, and save time for their own development. Unlike large frameworks, toolkits provide these services with light-weight interfaces and little or no restriction on application data structures, making them easy to use in both new and existing applications. In this paper, we describe features of effective toolkit design, using the Zoltan parallel, dynamic data management toolkit as an example.