Despite recent efforts in opening up government data, developing tools for taxpayers to make sense of extensive and multi-faceted budget data remains an open challenge. In this paper, we present BudgetMap, an issue-driven classification and navigation interface for the budgets of government programs. Our novel issue-driven approach can complement the traditional budget classification system used by government organizations by reflecting time-evolving public interests. BudgetMap elicits the public to tag government programs with social issues by providing two modes of tagging. User-initiated tagging allows people to voluntarily search for programs of interest and classify each program with related social issues, while system-initiated tagging guides people through possible matches of issues and programs via microtasks. BudgetMap then facilitates visual exploration of the tagged budget data. Our evaluation shows that participants' awareness and understanding of budgetary issues increased after using BudgetMap, while they collaboratively identified issue-budget links with quality comparable to expert-generated links.