Don’t be shy…
(not sure if this is better placed in IMHO, but…)
Several members have noted in posts that they have written or edited Wikipedia articles. KellyM is quite deeply involved in the Wiki “movement,” and can probably furnish you with much more information.
I’ve made a handful of minor corrections and additions to entries.
I’ve seen submission by “ybeafy” (IIR his username correctly) and I’ve always assumed it was the same as the poster here by that name.
This will probably be moved to IMHO.
I’ve contributed several edits and articles. Probably most notably, I started the Steak N Shake article (which has since been moved…my name’s only at the bottom of this page, which is the original title I started it under).
I cleaned up some “graffiti” on one article.
I do a lot of minor edits, and have started a handful of articles.
I modified one word, once… 
I don’t recall starting any articles, but I’ve added a paragraph or two to maybe a dozen articles.
Oh, wait, I did start the article about the 2001: A Space Odyssey comic book.
–Cliffy
Myself being the least prolific among them. I submitted what was the entire Spiderman article to Uncyclopedia.
Also, I was first on the scene, “like a bull in a China Shop” to edit out the “Sean Penn is an Incompetent!!!1111!!!~!” pictures of the entry in wikipedia, and replace them with an actual paraphrase of what happened. It has since been polished beyond what I was capable of. I am kinda proud.
Perhaps this list will be of some interest. In addition, there are lots of us who may occasionally edit an article or two, but aren’t really regular contributors.
I’ve made some contributions to Wikipedia. You can find my user page here I wrote a couple short articles about San Diego sports earlier this year; made some edits on other San Diego-related articles; NPOVed some articles about cults (or “NRMs” if you like); contributed some information on the media coverage of the recent Summit of the Americas; made contributions to articles about the 1918 World Series and the modern wild card in playoffs; recently, made a small number of contributions and fixes to various articles regarding Judaism, Ladino (the language of the Spanish Jews) and some other linguistics things. I do a pretty fair number of grammar and NPOV edits while I’m clicking around. I guess you’re not asking for a full list, but you can have it here if you want, anyway.
I didn’t know about the Teemings Millions list–thanks Eurograff!
A professor of mine is having the students do an interesting assignment: write about a topic (germane to the class material) that is not covered well or at all on Wikipedia, and upload it to Wikipedia yourself. Great idea – one that I’ll incorporate into a class I teach. BTW, he asks us to send him a copy of what you wrote, in case it is changed or erased by the time he has a chance to see it online.
I expanded and clarified the article on GSM frequency ranges. 
Yeah, changed a bit here, added a bit there, mainly in the field of English Lit. I’m far too lazy to get deeply involved.
I’ve added a few things. Corrected a few and expanded a few. Baseball & Sci-Fi mainly. I also sent a check.
Jim
Of course, there was that whole ugly incident with regards to The Andy Griffith Show.
This is my article.
I’m in Wikipedia, but I don’t know why or how I came to be there. What’s written is vaguely accurate but very misleading and out of date. And I’m only a stub. I’m not sure if I want to be more than a stub (I can’t see the point) but if I did I wouldn’t know how to achieve this.
I have made minor edits. I am a registered user. I intend to write a wikibook.