NClass Alternatives

yEd is a free desktop application to quickly create, import, edit, and automatically arrange diagrams. It runs on Windows, Mac OS X, and Unix/Linux.

Online diagramming application

Open Source

LucidChart is the missing link in online productivity suites. LucidChart allows users to create, collaborate on, and publish attractive flowcharts and other diagrams from a web browser.

OmniGraffle is for creating precise graphics like website wireframes, an electrical system designs, or mapping out software class.

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

UMLGraph is a professional automated drawing tool that allows the designers the declarative specification and drawing of UML class and sequence diagram.

PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.

Open Source

StarUML

UMLet - Free UML Tool for Fast UML Diagrams

ArgoUML, free and safe download. ArgoUML latest version: Open-source diagramming application for personal computers. ArgoUML is a program that allows users to create complex and professional diagrams.

Lightweight, powerful, easy to use, UML tool

freemium €59.0 / One-off (Standard Edition)

Umbrello UML Modeller is a Unified Modelling Language diagram programme for KDE.

Diagram Designer