When I was first asked to replicate KPI Indicator functionality in WSS I thought it can't be too hard. I'd figured I'd be able to get a calculated field to output an IMG tag with a reference to a Site Collections image folder. NOT.
After hours of workflow hell and finding out that lists with SPD workflows don't like being templated and copied I came across a new field type created by Stefan Costan called IconSet.
IconSet simply offers a formula box where the result gets evaluated against three criteria to display one of three graphics. Stefan has included about 5 icon sets to start with. IconSet uses a third party formula engine which offers a pretty good coverage of Excel formulas which is basically the same set used in SharePoint.
Basically an excellent tool.
Installation was reasonably error free. I had some issues moving the dll's in to the GAC as I don't have VS installed on the production box. A little googling and an installation of the .NET v2 SDK and all was fine. I also took the liberty of overwriting a copy of Stefan's included icons with some images users preferred. One of the images was a blank image which basically represents a no value result or when no icon is wanted. This extends Stefan's solution a little.
Although the formula engine appears to be full featured is was unaware of SharePoint's [Today] or [Me] keywords. These won't work natively in a calculated field anyway so not much of a loss. The problem is the normal workaround of creating a fake Today field of any field type still did not allow the [Today] keyword to be used like you can in a calculated field. This meant creating intermediary calculated columns to handle some formulas with SharePoint specific formulas.
** UPDATE **
I've had a chance to look around a little and found a much better way of achieving this and will blog here soon.