Occasionally (or often), when one sends you Office 2007 files (docx, xlsx, pptx) and you are using OpenOffice, you wonder how to open it? I can suggest, my tried and tested solution on OpenOffice, on Linux.
The closest to Visio in Linux is OpenOffice Draw. This is, going by the convention that a diagram tool is an integral part of an office or productivity suite.
I have myself done two comprehensive SRS (System Requirement Specification) documents – about 50 pages in length and full of UI diagrams – using OpenOffice Draw. The connectors, the basic shapes and numerous the export options (or file formats) are all indispensable. But what was missing was a clipart of common shapes and elements.