4.7-inch iPhone 6 and the 5.5-inch iPhone 6 Plus
Free iPhone SIM Unlocking Software Available (iUnlock)
The software is being mirrored (Gizmodo mirror) by a number of locations along with downloadable source code. Tutorials are still being developed but some early instructions have become available. At this time the instructions are not very user-friendly, so casual users may want to wait until a more automated solution is published.
More download links and information has been compiled in this forum thread by ascham87.
A few commercial solutions had previously been made available, ranging from $50-$100 per unlock.