It's easy to drill a hole at a perfect 90 degree angle when you have a drill press, but for those of you who don't have a drill press, you can use a drill jig guide. Even for those woodworkers who do have a drill press, sometimes you just can't get your piece into proper position for whatever reason, a drill guide for a hand held drill may come in handy.
A drill guide fits over your hand held drill and lets you drill at a perfect right angle. When you are predrilling a screw for hardware, it is not usually necessary for it to be at a perfect angle, you can usually eyeball it. But if you need to drill a hole for inserting a dowel, or making a recess with a forstner bit, you need that hole to be as accurate as possible.
Below you can see a few versions of a drill guide. The first, from Rockler Hardware is designed to be used with forstener bits and does it's job very well. It only drills at 90 degree angles however. It costs about $16 dollars.
Another 90 degree only drill guide is made by Lee Valley. Pictured below, it comes with four bushings with sizes of 3/16, 1/4, 5/16, and 3/8 inches. This particular model guide is best suited to be used on dowels and cylindrical wood stock. As an added feature, it is the only jig that enables you to drill a hole on the corner edge of a board. Listed price is $20.
There is also a more versatile drill guide sold by Rockler priced at $50 dollars. This one offers the ability to use a wider range of drill bits, up to 2 1/4 inches in diameter. Here's a link to the manual for more details. It also is adjustable in angle to 45 degrees.
A comparable drill guide from Woodcraft is a bit more expensive at about 55$. As you can see, you can put many different sorts of bits in the chuck from 0-3/8", not only forstener bits.
In addition, it is adjustable, so that you can drill at any angle between 45 and 90 degrees. The depth stop lets you drill to a premeasured depth every time.
As well, (although I have not used this feature), you can drill round stock by using the V guides it comes with.
Are the added features worth the extra cost? Well, if you don't have a drill press, definitely, especially if you don't room for a dedicated drill press in your workshop.
The main benefit is that these drill guides are portable. If you are on the job site, or if you have glued up your project already and forgot to predrill the pieces on the drill press, and accuracy is very important, then a drill guide is a worthwhile piece of woodworking equipment to have.
Overall, these are versatile jigs that let you replace a drill press for mostly out of the shop tasks or for larger pieces that won't fit under the drill press. Obviously they will not be as sturdy or accurate as a real drill press, so keep that in mind when designing a project.