cheat sheets.

Logging:
  console.log("message" [,objects])
  console.debug("message" [,objects])
  console.info("message" [,objects])
  console.warn("message" [,objects])
  console.error("message" [,objects])

Assertions:
  console.assert(a, "message" [,objects])
  console.assertEquals(a, b, "message" [,objects])
  console.assertNotEquals(a, b, "message" [,objects])
  console.assertGreater(a, b, "message" [,objects])
  console.assertNotGreater(a, b, "message" [,objects])
  console.assertLess(a, b, "message" [,objects])
  console.assertNotLess(a, b, "message" [,objects])
  console.assertContains(a, b, "message" [,objects])
  console.assertNotContains(a, b, "message" [,objects])
  console.assertTrue(a, "message" [,objects])
  console.assertFalse(a, "message" [,objects])
  console.assertNull(a, "message" [,objects])
  console.assertNotNull(a, "message" [,objects])
  console.assertUndefined(a, "message" [,objects])
  console.assertNotUndefined(a, "message" [,objects])
  console.assertInstanceOf(a, b, "message" [,objects])
  console.assertNotInstanceOf(a, b, "message" [,objects])
  console.assertTypeOf(a, b, "message" [,objects])
  console.assertNotTypeOf(a, b, "message" [,objects])

Measurement:
  console.trace()
  console.time("name")
  console.timeEnd("name")
  console.count("name")

String Formatting:
  %s            string
  %d,%i,%l,%f   number
  %o            link to inspector
  %1.o          interactive table of properties
  %.o           array of property names
  %x            XML
  %N.x          XML with N levels expanded

Command Line:
  $("id")          document.getElementById()
  $$("css")        array of elements matching css selector
  $x("xpath")      array of elements matching xpath
  $0               last inspected object
  $1               next-to-last inspected object
  $n(N)            N-th from last inspected object
  inspect(object)  display object in inspector
  dir(object)      array of property names for object
  clear()          clear console

http://www.joehewitt.com/software/firebug/