If a company isn't hiring currently but you'd like to get more information about when they might be hiring, send them this letter.
1234, Main Street
Boston, MA 02123
04 April, 2005
<Recipient Address Goes Here>
I'm not sure if you're currently hiring, but I thought I would drop you a line to inquire about future hiring. I'm a <your line of work>, and I would love to work for <company name> sometime down the line.
I would be interested in learning of any positions <company name> might have opening up that would be appropriate for someone with my qualifications. Please let me know if I should send you a resume and any other pertinent information.
Thank you very much for your time.
encl: <List of enclosed items goes here>