Hey, just to throw ourselves in the ring: We're Chart.io, a Y Combinator company. Our product is "Google Analytics for databases" and we've basically stripped out a lot of the complexity of BI (and a lot of the cost :)
We're in private beta now, so email me at dbeyer at chart dot io if you'd like to give it a shot.
+1 for Chart.io. We use it and love it. It addresses the most costly part of BI tools which someone pointed out above - paying one of your engineers to hook up your data with an external tool or have to code something custom from scratch.
Nice article. Other than open source, one more affordable option available today is SAAS BI. Especially for small and medium companies who may not want to allocate precious IT resources for installing and maintaining the open source solutions in-house. I work for Zoho and we have one such SAAS BI solution, Zoho Reports (http://reports.zoho.com). Do have a look.
We're in private beta now, so email me at dbeyer at chart dot io if you'd like to give it a shot.