Nomnoml

Nomnoml draws class-style UML from a very small syntax: names in brackets, arrows between them. It has a distinctive look and gets out of the way.

It suits sketching a class model quickly, where full UML would slow you down.

#What it is good at

Quick class and package sketches during design.
Teaching material where the syntax should not be a distraction.
Diagrams embedded in code comments and READMEs.

#What it draws

Diagram	Syntax starts with
Class	[ClassName]
Association	[A] -> [B]
Inheritance	[A] <:- [B]
Composition	[A] +-> [B]
Compartments	[Name\|field;field\|method()]
Directives	#direction: right

#A worked example

Classes with compartments and inheritance:

The source, which you can paste into a new diagram and edit:

#direction: down
#spacing: 40

[Payment|
  amount: Money;
  currency: string|
  authorise();
  capture()
]

[Payment] <:- [CardPayment]
[Payment] <:- [BankTransfer]
[CardPayment] +-> [Card|
  last4: string;
  expiry: date
]
[Payment] -> [Ledger]

#A few things that catch people out

Directives at the top control layout: #direction, #spacing, #fill.
Pipes separate compartments, semicolons separate the lines inside one.
For full UML with sequence and activity diagrams, use PlantUML.

Vega

Full control when a standard chart is not enough.

Vega-Lite

A real chart in about ten lines.

Examples

Working diagrams you can copy.

Learn one properly. Borrow the rest when you need them.