I just can’t stop.
1. Induction materials
We’ve got one wiki page called “NewDeveloperInduction”, and on the day a new hire starts, they quickly learn that that page contains — or links to — all the basic information they’re going to need to know in their first week or two. We also have a policy that it’s the responsibility of any new person still on their probation (i.e first 3 months) to keep the induction pages up to date.
I mean this in quite a literal, physical sense. Years ago I left Australia to work in Canada, and though we didn’t have a wiki in the workplace there, we did have a shared webpage where all the expats maintained a list of information for new immigrants, including where to find the local public transit timetables, weather forecasts, tax information, and so forth. At my current job, we have a similar page for immigrants (with information on everything from radio stations to recommended banks), as well as one more focussed on the immediate geographical area around our office: car parking, places to get lunch, and so on.
Many wikis have plugins for polls/votes among users, but even if yours doesn’t, you can do it informally by saying “I think X. Who agrees/disagrees?” and letting people add their names to either list. We do it the informal way, and it works pretty well for us.
4. Soliciting help
Traditionally, if you didn’t know how to use something you’d either interrupt someone to ask them, or send round an email asking whether anyone could help you out. At my current work, we have a series of wiki pages called “FooTips” for various values of Foo, eg. “Linux”, “Perl”, etc. If you’re wondering how to achieve some particular thing and don’t really want to interrupt anyone, just add your question to the “Tips wanted” at the bottom of the page, and someone will probably pick it up via RecentChanges and answer it.
5. Professional development