I got another head's up, this time from Marco at Groeneveld Management Services, about another open source OLAP application at www.palo.net, below is the Overview from their website:
Are you working with big Excel workbooks that are difficult to maintain because of their size? Then Palo is for you. Palo is an advanced data store for Microsoft Excel that allows you to handle large amounts of Excel data on a small number of worksheets. In addition, it also allows you to share Excel data real-time with your collegues.
This Palo-enabled Excel sheet holds the data of more than 1000 conventional Excel sheets (click to enlarge)
More technically spoken: Palo is a high-capacity and high-speed database made especially for Microsoft Excel. It is multidimensional, memory based, able to consolidate data hierarchies in real time and supports write-back of data (MOLAP). Palo integrates seamlessly into your existing spreadsheets. Database access is done through simple spreadsheet formulas, so you don't loose anything of Excel's flexibility. more...
Palo Server is Open-Source, the Excel Add-In is free. So give it a try! Download...
Palo is not bound to Microsoft Excel. You can connect to Palo by using a variety of software APIs (C, Java, PHP, .NET).
If I have to analyse a small amount of data I will often resort to standard pivot tables in Excel as I feel very comfortable with manipulating data in this manner. I don't know if this is the sort of tool I would be able to use on a client project, but if it is any good it could definitely earn a spot in my toolbox