The article states: "Twitter makes its code available to outside developers"

I understand that this article targets people other than developers also, so they've not used anything technical like "API", but to make more sense, shouldn't that read:

"Twitter makes its data available to outside developers"

It's not just the data, it's the ability to push to the platform. I like the phrase "open tools" in place of API.

An API is making your code available for others to call, but this phrasing could be confused with open source.

This Squidoo lens is probably the most comprehensive list of Twitter apps on the web:

http://www.squidoo.com/twitterapps

For even more check out: http://twitdom.com & http://twtbase.com

Hm, I did a quick search, and it's missing all of the payment engines, and doesn't list popular Twitter URL shorteners like bit.ly