If you won't want to make a full-fledged UML diagram that can be read and understood by all kinds of tooling, but just a picture that looks like UML, I can hearily recommend yEd.
yEd is a free (beer, not speech) application that allows you to draw any graph-like diagram. It's an order of magnitude faster and simpler in use than any other diagramming tool I know, because you can only make graphs with it. But when you think about it, nearly all diagrams (e.g. in the UML) are graphs.
For example, yEd has a "UML class" node type which is simply a rectangle with a name and two multiline text fields (one for attributes, one for fields). This is horrible if you want to generate code from your class diagram, or if you want the same UML model to be presented in multiple views, but for plain pictures, it's just perfect.
This approach also allows you to relax some UML syntax a bit here and there. For example, I like to write my fields and methods on UML classes like
int MyField instead of the official UML-styled
MyField: Integer since after all, that's how I'd write it in the target programming language. UML purists would scream in terror, but I want to communicate a design to my peers, not win a syntax contest.
(Note: making non-graph-ish diagrams with yEd is possible, but painful. Notably, UML sequence diagrams are not a good fit. But those can be created excellently from text again with tools like PlantUML and MSCgen, which in turn suck a graph layouting)
(Note 2: I get nothing for promoting this tool like this. I'm just a fanboy)