A lot of people are really smug and say "Sorry my NDA won't let me". This is a clever way to get people to admit they never code anything for fun, and as far as I'm concerned, that is a *very* damning statement about an engineer.

Furthermore, even if you didn't have an NDA, a 200 line snippet of commercial code is rarely very interesting at all. The code for "confirm empty recycle bin" isn't getting you hired.

My advice:
* Solve an algorithmically difficult problem efficiently. Here's a site with a few examples: http://www.itasoftware.com/careers/jlisting.html?jid=20
* Include integrated test cases. Smart programmers know they make mistakes. Make it dreadfully obvious that your program is correct.
* Don't be too clever. Demonstrate that a 1% performance improvement must justify any incomprehensibility that it introduces. One thing that *really* pisses me off is when candidates write optimizations that any reasonable optimizing compiler would do.
* Have good comments.
* No compiler warnings if you want the job. Keep in mind they may compile it in a different environment.
* If you have the time, make it tangentially related to the company -- a small opengl demo for a game company, a graph algorithm for a search engine company, something tricky in c for an os company, etc.
* Don't write it in Java, even if it's a Java job. Monkeys can write Java.
* Be elegant with formatting. It doesn't really matter what spacing/indenting standard you follow, but be consistent.
* Don't pretty-print it on your website. Provide it in compiler-friendly format.

Also, as far as stealing open source code and claiming you wrote it ... if they suspect you're full of shit, they'll do a google search for it, and almost all open source code gets pretty-printed onto websites these days, and that sort of thing is an instant rejection.