Allot of websites use pop-up windows, or more generally cause the browser to open a new window.
The most common use is when linking out to an external website. The theory of opening a new window is that when the website visitor has finished with the external website, they just close the window and will return to the original website.
But this theory does assume that the website visitor wanted to return to the original website. If every website did this, then the visitors will become slowly overwhelmed with browser windows as they jump between websites, most of which are no longer relevant and are just consuming system resources. This becomes more of a problem on Windows computers that ironically do not handle large numbers of windows particularly well, primarily due to the task-bar quickly becoming too full, making the switching between windows particularly difficult for the average user.
Continuing with the typical Windows user, running on a 1024x768 display, most of the time they maximise each browser window, presumably this is because they can only focus on one window at a time, but this does cause a problem with them potentially not noticing a new window being opened. Because they did not request a new window, they should assume that they need to use the back button in their browser to return to the original website. However they will find that is is "broken" (disabled), fortunately most visitors have experienced this strange behaviour before, so they take the "leap of faith" of closing the browser window.
Taking that most search engine results pages link to external websites without the use of opening a new browser window, then perhaps all websites should use the same convention. Its not as though the majority of browsers don't allow new windows to be created on request.
Perhaps the most common example of needing to show extra information is with the terms and conditions found on a form.
Some website designers don't want to visually show a large block of text on a form and prefer to have a simple link to a terms page. However it is not really possible to use a traditional link as some browsers may forget any values entered into the form when they go onto a new page.
A quick and dirty solution is to open a small pop-up window with all the usual browser controls removed (e.g. the back button and address bar). It must be setup in such a way that the user cannot maximise it (potential accessibility issue), this is to show that a pop-up was used, and that they can safely close it.
A second potential solution does require more work but should work in most browsers. The terms get included in the page and are styled up to look acceptable, then some JavaScript is created that hides the content and creates a tool (anchor link) which displays the terms when the website visitor requests them. This solution is not perfect, for example some screen readers might not notice the apparently "new" content (like everything else, it will require testing), and there is still a "leap of faith" by the web-savvy visitors who wonder if the link will take them to a new page and loose the values they have already filled out in the form (perhaps a little down arrow next to the link could hint that the content will be exposed below).
However, perhaps the best solution is to just display the terms. In theory every visitor using the form should agree to them, so they should all see and read it, so why try to hide it?
Now the main problem with pop-ups, most visitors are getting pretty good with closing browser windows, this is mostly due to adverts polluting their browsing experience.
Most visitors with a modern, decent, browser have a pop-up blocker enabled by default. This saves the website visitor from having to always be ready to kill that next pop-up.
It is because of these tools that some visitors have completely forgotten about the pain pop-ups can cause. Now when they come across a website that requires pop-ups, they usually think the website is "broken" when a link does not appear to do anything. This is typically the case with people who have bought those cheap shareware pop-up blockers that simply look for and close new windows.
In quite a few cases there are better alternatives to creating new windows, however there will be the odd case where there is simply no other way, in these cases I use a generic pop-up script that uses unobtrusive JavaScript, as it falls back to a standard link when the required JavaScript for the pop-up is not available.
Any feedback would be greatly appreciated, I don't include comments due to the admin time required, but if you email me, I will reply and make appropriate updates. Also, if you would like to take a copy of this article, please read the terms this article is released under. This article was originally written Wednesday 6th December 2006.