Hi Kristin:
Do you own the Adagio Gridview module? I don't know of dedicated forecasting tools (though I'm sure they exist) however I don't think you need one. And getting the information into such a tool would take a bit of work.

With Gridview you could build a report that would should show each item as a row. You could then define columns to show sales. The columns could be monthly sales figures or quarterly. Other columns would be "On Hand", "On Sales Order", "On P/O" and "Days Lead Time" (the latter could be assigned to each item using an optional field). You could then add columns that show calculations such as "Days of stock remaining", "% Change" and so on. Or you could export to Excel and apply more complex calculations there.

The starting point is that you need to have a calculation (or set of calculations) that you use to determine order quantities and timing. Once you have those in place, then you can systemize them into GridView and Excel.