Recommendations for column-oriented database

I've found databases typically come in two flavors, your traditional row-oriented RDBMS or an object oriented database (OODBMS). However, in the mid 90s I remember, a new breed of databases showing up that were column oriented. Some of these were given the term 4GL, but I don't think it was a term that stuck.

What I'd like to know is the following:

• What column oriented databases still exist?
• What are the performance characteristics of these databases?
• Are there any open source column oriented databases?
• What platforms do they interoperate with (.NET, Java, etc)
• What's been your general experience with them?

The two column oriented databases that I remember working with are FAME and KDB.

HBase is an open-source column-oriented database system modelled on Google's BigTable.