UML Designer - UML Designer provides a set of common diagrams to work with UML 2.5 models.
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.
Diagramo - Diagramo is an online flowchart software - diagrams (create, edit and share)
draw.io - Online diagramming application
ARIS Express - ARIS Express is based on the proven ARIS methodology and industry standards.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
They are different animals. It's important to understand that Visio, Lucidchart etc. are drawing tools, while Modelio is Modelling tool. If you are doing modelling, use Modelio.
I recommend - Modelio.