I haven't mentioned this in a couple of years, so new readers might be interested in learning about how foundationless frames help deter varroa mites.  The absence of foundation can also help promote the health of your bees since chemicals are often embedded in the beeswax that's used to make foundation.  The infrastructure to help bees build regimented combs of honey without frames of foundation is generally referred to as a top bar.

Now that you know why you might choose top bars, let's talk about the specific frame Mike was asking about.  The photo at the top of this post shows the top bars that came with our premade Warre hive.  You can get these for $1.50 apiece at Beethinking (or at least I think you can --- that's where ours came from in 2012, although the picture on their website shows a different design).  Even though the top bar is only a tiny piece of wood, I think these are worth buying if you can afford them since they're rather complex to build yourself, and this design definitely helped our bees create beautiful, straight comb with no foundation.  We did modify the box slightly to use pins instead of nails, though, in order to make the frames easier to move.

I've seen (and used) various other top bar options, and one of these might be better for you, depending on your situation.  If you're using a Langstroth hive, it's simplest to modify the frames that come with the hive.  You can nail the wooden strip that's usually used to hold foundation on vertically instead of horizontally, but I didn't have luck with getting bees to build from that option.  Instead, thin strips of foundation at the top of each frame worked well for me.  (Of course, you still end up with a bit of foundation in the hive with this method, but much less than if you filled the whole frame with foundation.)

This image, through a viewing window up into a horizontal top-bar hive, shows the V-shaped top bars that are perhaps the simplest to construct if you're making your own.  The package we installed in our top bar hive absconded, so I can't tell you whether our bees would have built off these top bars, but I believe most top-bar beekeepers use a similar design with good results, so it's worth a try.

No matter what kind of top bar you use, the idea is pretty simple.  You want to make the bottom of the bar come to a point to tempt your bees to draw out comb in a straight line.  And you want to allow the right amount of space between frames (if you're using a vertical hive like a Warre hive or Langstroth hive) so that bees can pass through but won't be tempted to fill in the gap.  In contrast, with a horizontal top-bar hive, you want to have the top bars completey fill the space so that bees don't end up under the roof.  Those two design points in place, a top bar can be pretty much anything you dream up.