• Twilio
  • free tier for devs, though it's a trial with a one-time credit of $30
  • NoMoRobo and OpenVBX are based on this
  • Twilio Connect allows third-party apps (see here) to bill the end-user, rather than billing the developer!
• Plivo (free tier for devs)
• Tropo.com (free tier for devs)
• Sinch

Cloud telephony providers that are oriented towards end-users:

• Google Voice
• NoMoRobo
• OpenVBX (it is not turnkey like the others here — some assembly is required, though Standing Cloud and Dreamhost offer one-click installs)