we have an org of around 300 people and certain people are very good at sharing articles, tips, blogs, etc but it usually happens within sub teams (between 5-15 people). whats the best way to scale this up to facilitate a culture of collaboration across a larger set of folks.
From my experience, forced = hated. So you have to make people want to use it, ie make it useful. A central Wiki sounds like the best solution, but it's hard to say. You might want to look into MediaWiki, Traq, or Sharepoint Services (not to be confused with Office Sharepoint).
Your organization may find it encouraging if you post a list of the top contributors, editors, or visitors to the site. But that depends on how your org perceives competition.
You have to create an culture in which sharing is rewarded.
Post to central WIKI instead of email links.
Reward contributors and encourage bottom up organic collaboration
"Force" collaboration top down. By "force" you mean reward and encourage.
You must do all of this. And more.
You must teach collaboration
You must assure that all managers value and reward collaboration
You must measure collaboration.
Even then, you'll probably have to do even more.
I wouldn't suggest a central wiki for collaboration (aside from internal specific stuff). But for sharing information found online you should encourage people to use one of the many existing systems for this. Google Reader has a really nice sharing and commenting mechanism. Delicious would also be a good fit for what you want.
There's no reason to try to create a walled garden inside your organization for content that is being created outside of it. The system you create will not be as good as the ones that already exist and that will kill adoption.