<p>I work on a complex application where different teams work on their own modules with a degree of overlap. A while back we got a Mediawiki instance set up, partly at my prompting. I have a hard job getting people to actually use it, let alone contribute.</p>

<p>I can see a lot of benefit in sharing information. It may at least reduce the times we reinvent the wheel.</p>

<p>The wiki is not very structured, but I'm not sure that is a problem as long as you can search for what you need.</p>

<p>Any hints?</p>

Sell the idea of using the wiki to the developers. You've identified some benefits, share those with the developers. If they can see that they'll get something of value out of it they'll start using it.
Example advantages from
What Is a Wiki
Good for writing down quick ideas or longer ones, giving you more time for formal writing and editing.
Instantly collaborative without emailing documents, keeping the group in sync.
Accessible from anywhere with a web connection (if you don't mind writing in web-browser text forms).
Your archive, because every page revision is kept.
Exciting, immediate, and empowering--everyone has a say.