Shown below is a simple schematic created with CircuitLab, which is a drawing and simulation package referred to in a number of answers above. It appears that CircuitLab recently (October 2016) became available in arduino.stackexchange.
To use it, click on the circuit icon next to the image icon in the editing tools bar (at top of edit box).
When you edit a question or answer containing a CircuitLab schematic, the schematic can be edited also. The editor's components list contains numerous passive components, some generic transistors and op amps, and a few ICs (eg NE555). Apparently, to draw an Arduino is a fairly lengthy process starting with a Custom box (from the Annotation section). However, if one does get drawn and saved, it can be reused. From the FAQ: “How can I save my custom part settings for re-use on another schematic? ... you can copy and paste an item from one browser window to another and ... all of its parameters will be copied and pasted...”
CircuitLab documentation is available at circuitlab.com. Note, the CircuitLab editor and simulator does not work in all browsers; see CircuitLab Documentation: System Requirements for more information. Briefly: “CircuitLab officially supports Google Chrome and Mozilla Firefox”. While Apple Safari and Opera are not officially supported, users report success.
simulate this circuit – Schematic created using CircuitLab
Note, answers to “Embedding a schematic editor” on electronics.stackexchange meta include comments similar to above; replies from CircuitLab developers; and a couple of alternatives that now are moot. Several non-embedded schematics programs are listed in “Suggestions for a program for drawing simple schematics”.