Nobody likes complainers and interviewers often assume that some day you may be complaining about them if they hired you.
Speculative letters ask about future possibilities rather than a specific advertised job.
Such letters should be addressed by name to the head of a particular department or division in a company. State that you're interested in an appointment to talk about future possibilities for positions. It's often best to then follow up in a couple of days with a phone call. Whether you are having friends write references or whether you are receiving them, look for specifics that tie the person to the skills needed in the job. Also look for the overall picture created by the reference letters as a whole.