Pencil and paper
My personal favorite way of creating a UML diagram is with this "antiquated" technology.
It's a quick and dirty way of getting to it. It's easy to make revisions. Also, it's highly likely that you don't need to learn this pencil technology but you do have to be able to read your hand writing.
A good variation on this technology is a white board.
A drawing program for making diagrams. There are plugins for UML. For a basic license, it costs $99.
But you can also use this tool for drawing information architecture wireframes and for diagramming basic layouts for a mobile app and a host of other types of diagrams.
LucidChart is an online tool that has a free version and some a paid subscription. It has a lot of UML templates.
Gliffy is an online diagramming tool that requires a subscription.
Draw it up using Illustrator or the vector graphic tool of your choice! Drawing in Illustrator or another vector graphics tool will get the job done and if you know a tool already, then there is no learning curve!
This tool that is a plugin for Eclipse. This is a great tool if you want to auto-generate the database tables based on the diagrams.
SQL Power Architect
If you already have a database, this tool will help you reverse engineer its UML diagrams. This is also a tool that will automatically generate the SQL you need to create you database. You can then draw your diagrams, input each class's attributes and the tool will generate the SQL.