This is pretty normal (although not common, per se) and is generally a good thing. Good employers want to confirm what they see on a resume, because there's so much fluff out there. There are multiple ways of doing this: coding tests, open source, code samples, etc, and it's just a matter of personal preference of the hiring manager.

You have to see as a benefit to you (assuming you're competent), because:

1) You are much less likely to work with an idiot (most of them won't even submit a code sample, or it will be so bad it will be rejected), and

2) Since they can see the level of quality of your work, landing the job and negotiating a higher salary will be easier than just going on your resume alone, which everyone has

For awhile I was always unsure of what to send as a coding sample since most of the code I work was either for my day job or for contracting. I didn't feel comfortable "making up" a purpose for code. If that's you, you can try answering a public coding challenge and submitting it to the employer, like this one http://dotspots.com/jobs/challenges/