
The traditional bench-mounted vise, protruding as it does from the worksurface, is always in the way when you're not using it. And over time they can start to sag, which gets annoying. (The ones we had in the ID shop at Pratt did that thing where they got higher as you started to tighten them and lower as you loosened them.)
Workbench manufacturer Benchcrafted's Tail Vise is a clear design improvement. Integrated into the bench itself, the Tail Vise holds a dog block (that's on you to provide) meant to correspond with a row of dog holes in your bench.

One cool thing is that the screw doesn't move in and out of the table as it's turned, instead maintaining its position in space. Another cool feature is the cast-iron wheel, rather than a T-handle; the wheel affords greater precision and lets you more quickly crank a full revolution.

Here it is in action:
You probably won't be retrofitting this to your existing bench—unless you've figured out some clever way to cut stepped dog holes into your already-assembled work surface—but those starting from scratch will definitely want to look into one of these.

Comments
I recently finished building a workbench using the Benchcrafted hardware, and even though I created stepped / square dog holes (as seen in the video), retrofitting the hardware onto an existing bench really is a simple matter. You just need to drill circular dog holes into your pre-existing bench top, and make or buy circular dogs.
A typical benchtop vise has a purpose for it's shape. You won't be cutting pipes or metal bars on Benchcraft's vise, and after a few years of use, it's sure to get your benchtop shredded up with saw marks too.
Retrofitting their tail vice on existing benches is much easier than other tail/wagon vice retrofits, like Steve said round dogs are available but even less expensive is just boring round holes and squaring them up with hand chisels (because if you own a bench and have the cash to own a BC tail vice you certainly own nice chisels). I've built a roubo bench with both benchcrafted's tail and leg vice and its stellar hardware.
@Luke: You can avoid sawing directly into your benchtop by using a simple bench hook and hanging the waste-end of the board you're cutting off the edge of the benchtop. After a few years of use, I don't have *any* saw marks in my benchtop.