Suggestions for a program for drawing simple schematics
A compilation of possibilities mentioned the comments. I'll add to this as I try out the suggestions.

From @Gerben, "kind of in order of complexity":


circuitlab.com - online schematic capture and simulator (analog, digital, and mixed signal). Licensed for professional, student/educator, and hobbyist (free) usage. Officially runs in Chrome and Firefox, and they report users having success with Safari and Opera which are not officially supported.
Fritzing - cross-platform (Mac, Windows, Linux). Has Breadboard View mode with built in Arduino support that allows you to create diagrams like this one:



From there you can click on the Schematic view to route wires and convert the breadboard into a traditional schematic.
CircuitMaker - Windows only, requires creating an account.
Eagle - cross-platform (Mac, Windows, Linux), paid and free versions.

Component library collections (thanks to Marko Mackic for the idea):


Adafruit
Element14 Arduino library - requires registration, lots of other libraries as well
Sparkfun

KiCad - cross-platform (Mac, Linux (with packages for several distributions), Windows, and source code), open source and also on GitHub.


Found Autodesk's 123D Circuits in a question on Arduino Simulators (and emulators).