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Unfortunately that leaves some great niche products to struggle to gain a user base. Trying to find one language that is both powerful/expressive and supported across various platforms is the trick, the answer is an opinion ATM.

D-Wave's Leap and Ocean SDK allows access to a D-Wave 2000Q™ System in a cloud environment with access to a 2000+ qubit quantun annealing machine to test and run workloads for free, assuming the core algorithms used go into the open source pool. Apply to login at D-Wave's Leap In webpage.

Rigetti Computing's Quantum Cloud Service (QCS) offers a Quantum Machine Image, a virtualized programming and execution environment that is pre-configured with Forest 2.0, to access up to 16 qubits of a 128 qubit computer.

Stay tuned for information on Fujitsu's Digital Annealer, an architecture capable of performing computations some 10,000 times faster than a conventional computer. If they eventually provide a development environment that is cross compatible with true quantum computers these two paragraphs will remain in this answer, otherwise I will remove them.

Unfortunately, that leaves some great niche products to struggle to gain a user base. Trying to find one language that is both powerful/expressive and supported across various platforms is the trick, the answer is an opinion ATM.

D-Wave's Leap and Ocean SDK allows access to a D-Wave 2000Q™ System in a cloud environment with access to a 2000+ qubit quantum annealing machine to test and run workloads for free, assuming the core algorithms used go into the open source pool. Apply to login at D-Wave's Leap In webpage.

Rigetti Computing's Quantum Cloud Service (QCS) offers a Quantum Machine Image, a virtualized programming, and execution environment that is pre-configured with Forest 2.0, to access up to 16 qubits of a 128 qubit computer.

Stay tuned for information on Fujitsu's Digital Annealer, an architecture capable of performing computations some 10,000 times faster than a conventional computer. If they eventually provide a development environment that is cross-compatible with true quantum computers these two paragraphs will remain in this answer, otherwise I will remove them.

I will return to this answer from time to time to make updates, without excessive bumping.

A review of: Cirq, Cliffords.jl, dimod, dwave-system, FermiLib, Forest (pyQuil & Grove), OpenFermion, ProjectQ, PyZX, QGL.jl, Qbsolv, Qiskit Terra and Aqua, Qiskit Tutorials, and Qiskit.js, Qrack, Quantum Fog, Quantum++, Qubiter, Quirk, reference-qvm, ScaffCC, Strawberry Fields, XACC, and finally XACC VQE is offered in the paper: "Open source software in quantum computing" (Dec 21 2018), by Mark Fingerhuth, Tomáš Babej, and Peter Wittek.

I will return to this answer from time to time to make updates, without excessive bumping.

Updates:

Google's Cirq and OpenFermion-Cirq: "Google's AI Blog - Announcing Cirq: An Open Source Framework for NISQ Algorithms".

D-Wave's Leap and Ocean SDK allows access to a D-Wave 2000Q™ System in a cloud environment with access to a 2000+ qubit quantun annealing machine to test and run workloads for free, assuming the core algorithms used go into the open source pool. Apply to login at D-Wave's Leap In webpage.

Rigetti Computing's Quantum Cloud Service (QCS) offers a Quantum Machine Image, a virtualized programming and execution environment that is pre-configured with Forest 2.0, to access up to 16 qubits of a 128 qubit computer.

Stay tuned for information on Fujitsu's Digital Annealer, an architecture capable of performing computations some 10,000 times faster than a conventional computer. If they eventually provide a development environment that is cross compatible with true quantum computers these two paragraphs will remain in this answer, otherwise I will remove them.

While their silicon chip is not quantum in nature Fujitsu has partnered with 1Qbit to develop what is described as a "Quantum Inspired AI Cloud Service", whether their Digital Annealer quacks like a duck (anneals like a D-Wave, and uses compatible code) remains to be seen. Visit here to access the Fujitsu Digital Annealer Technical Service.

University of Pennsylvania's QWIRE (choir) is a quantum circuit language and formal verification tool, it has a GitHub webpage.