The folks over at iPhone Dev Wiki posted a free SIM unlock tool for the iPhone. Similar to the other solutions, this software-only solution should allow you to SIM unlock your iPhone, allowing you to use any GSM cell phone carrier. In the U.S. this limits your alternative choice to T-Mobile (over AT&T), but in other countries, GSM carriers are far more common.
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.