How can I generate a rendered preview of selected markdown text or files? I would like to be able to pop up a window on my Mac displaying a rendered HTML preview of selected text (ideally a completely chromeless window like that used by QuickLook, but an ordinary Web browser window could work if necessary, but the less chrome the better).

What tools are available to do this?
To view a HTML file (I'll leave you to work out exactly how you want to format it and parse the markdown etc) in a chromeless window similar to quicklook you can use the automator actions Get Urls From list > Safari Popup.

This will open the URL (which can be a local file://~/etc) in a box as follows (the following screenshot shows facebook - as you can also set window size and user agent in the safari popup options - I choice iPhone size and Mobile Safari agent to show how you can create a quick and dirty Facebook app using automator in 10 seconds).  Clicking OK or cancel closes the app as it's the last Automator step, but you could easily extend it so a canel stops the workflow, but OK continues the actions, which may me to copy the file to an upload folder or whatever.