There are a few solutions that i recommend and suggest you look into. Bloomfire, BMC Software, and Kana. Additionally, Salesforce has a KM module in their CRM suite. All of these can be accessed via the web and are hosted solutions (BMC Software offers an on-premise solution as well). They are all great choices and picking one depends on the needs/size of your company (Salesforce tends to be a bit more expensive than the others, but has the reliability and infrastructure to service enterprise-sized companies).