I use the free personal edition of Atlassian’s Confluence Enterprise Wiki for all my Document and Knowledge Management needs. It runs off my iMac at home and is a simply magnificent piece of software, but limited to three users, which is fine if it is just for you, or you and your partner.
I read today that they are now offering the 10 user hosted Team edition for $49 per month. This is an absolute bargain. We have the full enterprise edition at DNS Europe which cost us $4000 to buy and then $600 per annum thereafter.
If you run a small team or your company is is under 10 people in size, you should jump at this.