Are there any free open source software tools available for simulating Oxford Nanopore reads?
Simulators designed specifically for Oxford Nanopore:
General long read simulators:
For an exhaustive list of existing read simulators, see page 15 of my thesis, Novel computational techniques for mapping and classifying Next-Generation Sequencing data.
The best nanopore read simulators would be associated with the best base-callers. For a base-caller to effectively model the DNA strand, it needs to take into account the expected underlying electrical model together with the associated signal noise (both in the time dimension as well as the amplitude dimension). In theory, it should be possible to reverse the algorithm and generate an electrical signal given an underlying sequence.
Unfortunately, I'm not aware of any tools that attempt to simulate nanopore reads at the electrical level. Any "nanopore read simulator" that concentrates only on base sequence would need to encompass all the possible base-calling software models that exist, which is an impossible task (particularly given how quickly ONT updates their own base callers).