If you're really, really sweet to Miss Tracy (aka starl) in a PM she might give you some feedback on the most frequently-appearing kinds of projects Mr. Excel's consulting gurus get called in for. I would think that Tracy would have one of the best views of anyone anywhere as far as where Excel gets applied.
Kind of a different interpretation of your question -- it's not really an "application" per se. But I would have to say that Excel's most common usage is as a "generic list exchange" application. You can take any old list of whatevers -- part numbers, model numbers, invoice numbers, PO numbers, Airwaybill numbers -- and e-mail to vendors, customers, whomever and they can open it and use it; i.e. dump it into their system and/or run a query of some type -- Qty On-Hand, List Prices, Invoices Open, PO ETA's, Tracking Info; and then return it to you with the data you were needed appended to it. That's probably how most of my co-workers use Excel. Only folks that get beyond the novice stage use it to "develop an application" or to crank meaningful analyses.<hr />
Were they preparing their financials using tables in Word?
Back to the topic at hand (so as not to worry Erik ) -- I'm starting to play with its XML stuff a bit. While not a mainstream usage of it today, I would think that in the future that might be an area where Excel will get used more.