While designing my site I am constantly faced with the issue of whether its ok to TAKE ideas and designs from other sites. In some cases there is no distinction in certain aspec
It's a sliding scale. Borrowing just an idea is one thing, if you're incorporating it into the rest of your existing design, not just wholesale copying an idea. Snagging a idea for a design element is fine, copying a whole design exactly is not. As you borrow more and more of a design, it gets into the not acceptable category. Copying directly is also another factor. If you see something you like and reimplement it for yourself, that is typically fine. But doing a direct copy of code, images, or css not so much.
I'm not a web developer, but I might have some insight that will help as well. My team has created several applications that have served as the starting point for other applications delivered to various customers.
The successful derivatives were those in which the developers took the time to learn the architecture and why things were the way they were. They then took the more crusty parts and rewrote them and in general expanded and improved the architecture.
Invariably, when a team simply took the existing project and tried to 'brand it' or copy it for a customer without actually figuring out the systems, they either created poor implementations of the extensions or had the project fail outright.
I realize this is a bit off the main topic of the ethical issues address by others here just fine, but my bottom line is that pure theft usually costs you more time than it saves.
Depends on how much you 'steal'.
Code
If you're ripping off the whole design, then its a bit dodgy. If you like (for example) the Stack Overflow concept of voting up stuff, then steal the concept and use it in a different manner. If you want to know how say the orange highlighting of the up-voted items works, then look at the code. But don't do both and steal both the concept and the design, you'll just create a clone.
Due to the way different web browsers treat CSS and the like, there are often only a very few limited ways to do a particular thing (3-column layouts, etc.). It seems fair enough to blatantly copy in these cases where there is a common way of doing things. Where its something unique, and there's many ways of doing it, it seems a bit more off to blatantly copy.
Graphics
Ripping off graphics - not so okay. Images have been around a lot longer than code so copyright law, etc. probably suits them better. If nothing else you have to contend with possible watermarks or other metadata to identify the original source. It's very easy to check for image stealing, less so for code within a larger block.
I'm a coder, not a designer so what I tend to do is borrow graphics that I like just while mocking up my web-app for internal use. Does that seem fair? I'll change them for newly-designed or paid-for ones before going live. At least that's the idea, though it could be far too easy to forget and use them by accident.
That's the way it works in the newspaper world (well it used to, not sure now with the advent of this there Internet thang): You download as many graphics as you can bother waiting to come over your 57.6k modem; you only pay for the ones you actually publish.
Stealing code or designs is immoral and in some cases illegal.
Taking inspiration or copying functionality is less of a problem. For example, at some point in time someone realized that putting a "Forgot Password?" link next to all login forms is a good idea, now everyone does it. It's not theft it's just replicating a good idea.
I think its ok to steal ideas, but not to steal code.
I am a web developer, not a designer. As such, I have a sense of taste, but not the ability to come up with something wholly on my own. As a matter of ethics, everything commercial or with the expectation of serious traffic that I do, I will hire a designer. They need to eat too, and there is something wrong with making money off of others work and not compensating them for it.
If it is small, personal, or an internal throwaway type thing, I will rip off things like color scheme and/or layout. Technically you could say this is stealing, but I think of it more as "imitation being the sincerest form of flattery" thing. I don't feel that bad about it since there isn't really any money to be made in it.