Pencil is a standalone app that lets you create diagrams or UI mockups. It doesn't have collaboration/multi-user features.

Dia is a GTK+ based diagram creation program for Linux, Unix, Mac OS X and Windows released under the GPL license.

yEd is a powerful diagram editor that can be used to quickly create diagrams manually or import external data for analysis and auto-magically arrange even large data sets by just pressing a button. You can use yEd to quickly create flow charts, BPMN and UML diagrams...

Create data-connected diagrams with real-time collaboration as part of the Office Suite. Ideal for IT and business professionals, it visualizes complex info with ease, providing intuitive tools for diagramming, mind mapping, and vector graphics.

Clickcharts flowchart software allows for easy drawing of diagrams and flowcharts to provide a way of organising and displaying data. Lay out your ideas, organisation and processes or create UML, Venn or mind map diagrams with this flow diagram maker.

MyDraw is an easy to use, yet powerful diagramming software that lets you create a large number of diagrams like flowcharts, organizational charts, mind maps, network diagrams, floor plans, family trees, business model diagrams, and many more.

Merge Plan is a user friendly diagram application. It is fully compatible with most diagram file formats. It is a great value in comparison to other diagram applications. It is part of Merge Soft Office.