I'm a pretty strong believer that anything related to the development should go on the project Wiki.

Most people will agree on a few obvious topics:

• Rules & standards
• Design decisions (which typically turns into documentation of some sort)
• Setting up the environment
• Development processes

I prefer if the Wiki was more a 'place for everybody'.  Because the goal is that you want to drive your team to firstly Read the Wiki and then secondly to Update the Wiki. 

This means it needs topics that are less hardcore and has a more personal touch:

• Bus / Train timetables - tips for getting home on time
• Good restaurants near work - with Google/MSN Maps integration
• I love this one - a Quotes page of who said what
• Office Foosball / Table Tennis championship scores (and champions)
• Team money jar summary
• Breaking the build fine
• Late/absent for scrum fine
• Swear jar
• Playing foosball / table tennis out of legal hours
• Wearing jeans to work (usually this goes to some sort of charity though)

When someone on the team has a problem, what do they do

1. Yell out help - can anyone help me?!
2. Or they search the wiki

On most projects, #1 happens all the time, and #2 pretty much never happens.  Which is a pity.