What is a good way to steam tamales without a "tamale steamer?"
Alton Brown recommends steaming them right in a normal tall pot with a steamer insert (your typical expanding/contracting one many people have on hand), directly in their husks.  Basically, you put a couple inches of water in, and then a steamer insert, and the tamales (in the husks) go on top of the steamer.

You can find his recipe/method here.

You can also watch the episode on YouTube, go to about 4:45 in to see the tamales in the pot.

If you don't have a steamer insert of any kind, I would imagine you could fashion something out of aluminum foil without much problem.  The most important thing is that you keep the tamales out of the water.  In a pinch, an upside-down colander would work if you have a pot that's large enough to hold it.

Another (very low-tech) option is this hack, that basically uses a disposable aluminum pan to create the steamer/upside-down colander part of the rig.  This seems like a pretty easy, straightforward solution.  

Edit: as Michael points out in the comments, this seems like it could be a huge mess.  I would recommend getting a disposable pie pan that's just smaller than your pot, poking holes in that, and putting it upside down in the pot like a steamer insert - seems a lot safer and more efficient than a large rectangle on top of the pot!