StarUML - StarUML
Playscii - Full-featured ASCII art and graphic editor and game creation tool
draw.io - Online diagramming application
UMLet - UMLet - Free UML Tool for Fast UML Diagrams
UMLGraph - UMLGraph is a professional automated drawing tool that allows the designers the declarative specification and drawing of UML class and sequence diagram.
LucidChart - 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.