If you have more than one feature and only one field, you will inevitably end up with conventions for smuggling extra data into it, which users will then have to learn. E.g., Twitter has @user and #tag, and Google has site:, inurl:, and cache:.

This way we can have our cake and eat it too. New users see only one input, making it very clear what they're supposed to do, and advanced users can still do advanced things.

Might be worth noting that @user and #tag were conventions invented by users and later adopted by Twitter itself

Chrome has 1 textfield that's both used for search and for urls.

Related: I've found the search textfield in the Windows 7 START menu hugely useful. Just type 2 or 3 letters instead of browsing through folders of programs. 1 textfield + autocomplete = powerful for known-item searching.

And: Twitterbar adds more functionality (tweet) to the URL bar of Firefox.

One textbar is definitely the way to go, with as only downside that learnability goes down a little :)

Well, technically they all have other 'stuff' to help you do whatever it is with that single box. Google is the only one that doesn't require you to signup before using it. Digg and HN kind of only have 1 input box.

It definitely is an interesting trend. I've even tried to come up with startup ideas restricting myself to simple inputs like a single textbox. Sometimes adding some constraint helps you be more creative.

I would focus less on the input box and more on the task. Each app focuses on a single task presented with as few options as possible. it's hard to use any service wrong because there is not much to mess up as a user.

Try to make a calendar app with only one text field. There is no way "one text field only" can be applied as a general rule, but just thinking about how you could apply it will (IMO) make your app better.

What the apps listed above have done is simplified the interface to the bare minimum needed to accomplish the task. Sometimes, that requires 5 text fields, sometimes 1.

The biggest value of a good user experience designer is in figuring out the minimal functionality for each step in the application and stringing those together in ways that make sense.

While not a calendar app, I used the "one text field only" methodology on Weekis - http://weekis.com which is a simple week planning tool.

Interesting...I originally had a more complicated interface on http://buyersvote.com/ where people could add/remove items on a list which would pop out a new text field for each item.

I eventually just made it one big text area and let them do "one per line".

There is a still a sparate field for the title. I don't have any hard data on whether this change improved engagement or not. But I think as a general rule, the simpler you can make the UI the more engagement you'll get from first time users.

Not to mention the page we're on now.

Questioning user interface dogma in your field can have interesting results. I'm building a DNS service at the moment and having experimented with a few user interfaces, have found the one I like the most has a single page for viewing and editing the zone, fixed record names and types, no class or TTL (at all), and a field for each unit in a records data, rather than the traditional one data field per record data. It's more limited than other interfaces but it's also more powerful; have to wait and see whether the market agrees.

xterm

When you think about it, the command line is the ultimate "single text field" app.

It all depends on the nature of your web app. Single-text-field UI doesn't really make sense for, say, an email composition app.

Friendfeed and Twitter each have two. One for search and one for posting.