Imagine that you have the next silver bullet software idea and you are a terrific programmer but you don't know a thing about Photoshop and colors stuff.
Do you hire a graphic? Do you steal from some free templates?
Do you have any advice to handle this common problem?
EDIT:
My question is on GUI in general: Web, Desktop or Mobile.
I guess this depends strongly on the type of user interface you want to create.
If you want to create a UI experience akin to e.g. Ableton Live, you'll most likely need some graphics / designer people. In the case of Ableton's Live, it shows that the UI designers knew what they were doing... it's an awesome UI.
If, on the other hand, you are creating some business software with a fairly standard UI (i.e. some forms, tables, data grids etc.), you'll probably be fine doing it yourself.
Build a command-line application and make it so your programs have an API that have enough hooks to allow other people to write a GUI application just from the API. Someone else would build the GUI for you. At least that have worked for a good many open-source applications, so open-sourcing your program might be a good idea as well.
Depends on what you need. If it's just icons for menu items and buttons, there are plenty of free resources out there and even one or two search engines. If it's a fully blown marketing website, you better hire someone who knows what he's doing.
From my experience, if you want professional-looking software (specially in the web), hire a pro designer.
Typically the design is not that important. Take an application like the terminal or Firefox: Users expect the standard colors and standard design elements. You can always look at other programs and let them inspire you.
But if, for some reason, colors and design are really important for your application and you don't know much about it then of course you should ask around and get a designer to do the job.
There are tons of premium templates you can by that wouldn't be neccesarily stealing as long as you pay for them. I know there are tons of Admin templates out there that look absolute amazing.
Consider implementing your software as a service. Perhaps a web service but even something that home baked with a socket based interface can do well.
Creating GUI applications doesn't always require graphics design. The buttons, checkboxes and scrollbars are built in to the OS. Of course you do need to determine how to lay them out, which can be very difficult to get right. I'd try to create the GUI in the programming language you're using, and then ask for criticism from other people. You can always decide to hire a professional GUI designer later. It will save you and him/her a lot of time if you already have a GUI. It's much easier to get your ideas across if you have a prototype to show.
Since you don't say if it's desktop or browser based, I'll take a stab at desktop.
I liked Lie Ryan's suggest to make it a command line program with a really good api, so that others can build a gui. A very unix-like approach, which also allows the input of other programs to pipe into yours and the output of your program to pipe into others.
If you want something extremely professional, Microsoft have spent hundred of millions watching how people use programs and refining their interface. Recommended reading is here http://msdn.microsoft.com/en-us/library/ms993296.aspx I guess that would go for platforms other than windows.
Now, that refers to design/layout of controls and, to some extent generic program operation. I am not sure if you want that, or just a program to help you build GUIs.
Whatever you do I would *strongly*advise you to keep your program like pothers - like people have gotten used to. E.G a "file" menu on the left, even if you don't do anything with files. It probably ought to have been called "application", but it's too late now, people expect "file". "Help menu on the right". Dialog boxes with Ok on the left (as default) and cancel on the right. Hot-keys for the rodent-phobic, etc, etc.
Now, if what you want is just a RAD GIU builder, then check out the free edition of Borland Delphi 7 if you like Pascal or are afraid of C+ (I personally prefer Borland products because of the thousands of free components available; I am unaware of any competitor with so many, but would be happy to be corrected).
If you are comfortable with C++, check out MS's free C++ RAD GUI at http://www.microsoft.com/express/Downloads/ or QtBuilder, which is free and cross-platform.
Of course, if you want cross-platform you can also use Java. In any case, look at NetBeans, which supports many languages.
And if you use Linux exclusively, I would recommend KDevelop, even if you use the GNOME desktop.
I may have answered the wrong question here, as the original was not very clear, but I hope that this has been of some use to someone.
Build the GUI so that it is functional. If it works well - simple, quick, intuitive - then that is the best UI you can hope for. If it's a successful idea then you can make it pretty later on.
If you need to make it pretty, then get a designer to help you. Either to give a design that you can then follow, or to actual develop the whole UI (which will be more expensive).
User Testing.
I cannot emphasize how necessary this is. Build a prototype GUI, put placeholders everywhere you need to, and get everyone you can to try it out. Listen to what they say, add or remove what they want, and repeat.
You can go very far on your own with tools such as Interface Builder or QT Creater. They help you follow some basic rules: how to space controls, buttons, text fields... It's super-easy to move things around, so you can iterate and find out an arrangement that is pleasing to the eye.
If that's still not good enough, try to get help from a designer. I'd try to find a friend of a friend, or a student. You don't have to spend $10K for a pro to fix your GUI, unless you can afford it.
I somewhat disagree with other advice saying that you should just provide a great CLI and let others add a GUI to it. Imagine how that advice would have played for iPhone touch-heavy apps :-) Unless you are designing a batch-mode video compression algorithm of course.