As great as laptops can be, they often lack something that many pros swear by on their desktops--a second monitor. Second monitors represent the ideal blend of functionality that comes from having two documents available full-screen for simultaneous reference. Multiple studies have shown that adding a second monitor can boost productivity.

While there have been some experiments with multiple monitor laptops, a single display is an understandable compromise as the whole purpose of a notebook computer is to enhance portability. And even flat-panel desktop displays tend to be somewhat bulky, with a multitude of ports a power cord that needs to be plugged in.

But nowadays, with pressure mounting from tablet competition, ultraslim notebooks screens are getting smaller than they've traditionally been. For example, Apple's very popular MacBook Air is offered only in 11" and 13" models. Most Windows PCs that offer detachable keyboards--like Microsoft's Surface--are also available in sizes below 13". Higher-resolution displays help, but require their own tradeoffs on a small screen.

However, a relatively little-known standard called DisplayLink can allow you to easily add a slim second monitor to your laptop using any available USB connector. In its early days, DisplayLink monitors were small novelties that were only practical for, say, keeping your chats on a second display. For the past few years, though, they've been available in 15" and larger sizes and may be able to double your laptop work space.

Indeed, two recent entrants include the 15.6" ASUS MB168B+ and the 17" AOC E1759Fwu. After loading drivers from the DisplayLink Web site (or the included CD), just plug in the monitor into a free USB port on your laptop and the new display will light up. Since power is drawn from the USB port, there's no need for an extra power cord, although you will probably want to keep your laptop plugged in while you have the monitor attached as it can drain the battery. You can set the display to extend your desktop or mirror it if you'd like to give a presentation while seeing what you're presenting. For long documents, you can even set up the monitor in a portrait orientation.

As for disadvantages, don't expect either model to blow you away with brightness or viewing angles. (One of DisplayLink's benefits is reduced power consumption, although the ASUS seemed to offer better contrast.) And while the cable they come with is technically a standard one, make sure you don't leave it behind--it is not commonly available.

Both models come with protective cases. In the case of the ASUS, the case--which is more stylish than the AOC's--also acts as a stand that works with the monitor in landscape or portrait orientation. The AOC monitor, on the other hand, has a kickstand that folds into its back. It also works in either orientation; this makes its profile a bit thicker even though the support is better than ASUS' case. The ASUS also has a prettier back with a subtle circular pattern that has been a mark of its laptops.

But the bigger difference is the resolution. Even though the ASUS monitor is smaller, it can fit more on the screen due to its higher resolution--1920 X 1080 ("full HD") vs. the AOC's 1600 X 900. And there's another feature the ASUS has over the AOC: an on-off button. With the AOC, you have to unplug the monitor to turn it off. Still, the ASUS may be a better choice for those who prefer on-screen elements to be a bit larger.

The ASUS lists for $259 while the AOC is $199. Both products also have less expensive siblings--the ASUS MB168B (an identical model designation save for the plus sign) has lower resolution (1366 x 768) available for about $165 while the AOC 1659Fwu comes in a 16" vs. 17" screen size and is the lowest-price option of the set. It's available on Amazon for about $132.