I publish under a pen name when I don't have any editorial control over the final product. It's not about being ashamed of low-quality work, but more about being embarrassed by errors that did not originate with me. I work in education, and if people Google me and find incorrect articles with my byline, it would definitely hurt me professionally.

Frequently, online publishers reorganize themselves and bylines get lost or mixed up in the restructuring. So even if you do use your real name, there's no guarantee it will stay on the piece forever, especially if you sell full rights.

Using a pseudonym won't prevent you from including such writing experience on a resume (should you want to) since you will have records to prove you worked for the company. Clips are another matter.

Just for fun, here are some random pseudonym generators:
http://www.kleimo.com/random/name.cfm
http://www.had2know.com/entertainmen...generator.html