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One key that many people miss when they design an information architecture is that, done well, the marketing purposes of the website should inform the IA. So it's rather difficult for someone outside an organization to know if any particular IA is good, or weak.
Another frequent IA error is this - within those marketing purposes, the best IA is designed from the user viewpoint, not the company's. That sounds like a no-brainer, but it isn't. Many times the website structure is created so that employees and various internal stakeholders can easily navigate to the paarts of the site that they are interested in. But that usually creates a kind of information jungle for the visitor, who really should be spoon-fed, almost, with the kind of information packaging that serves their needs.
So it's not an easy thing to give examples of what is a "good information architecture" without knowing the goals of the company, and the website metrics. In addition, the Forum Charter [webmasterworld.com] discourages example sites, so we can't go off in that direction.
We've got some threads in the library that are worth a read:
Information Architecture for the Small Site - part 1 [webmasterworld.com]
Information Architecture for the Small Site - part 2 [webmasterworld.com]
Putting information architecture into practice [webmasterworld.com]
And the best text I know of is the O'Reilly book Information Architecture for the Worl Wide Web. I's in its third edition now.