This has already been done several times, in several ways. When I get a chance I will either write yet another way, or find one of the other examples.
However, this entire approach has a few problems. Support for iframe is being phased out. Some browsers already support not allowing iframes. Even with an iframe available, many sites will not allow themselves to be displayed in an iframe. This isn't so bad when you, the designer get to pick the sites, but when the user can type in the addresses, there is no telling what sites will be chosen - therefore no way of knowing if these sites will allow themselves to appear in the iframe. Those that do not will take over the entire page, or at least try to (some users will find that when viewing this page on the internet over certain local networks not even that will be allowed, so a blank page may be displayed or some sort of error, or even a reprimand).
Try:
Code:
URL: <input type="text" id="theurl">
<input type="button" onclick="jumpto(document.getElementById('theurl').value);" value="Go!">
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