I recently noticed a very nice add-in developed by Bryan McKelvey called FinAnSu. The whole add-in is generously available under the MIT open source license, and is a fantastic example of what can be built with Excel-DNA.
FinAnSu uses a ribbon interface to make the various functions and macros easy to find. The RTD server support is used to implement asynchronous data update functions, providing a live quote feed from Bloomberg, Google or Yahoo! And then there is a bunch of useful-looking financial functions. Here's a little preview:
Find the project on Google code: http://code.google.com/p/finansu/, with detailed documentation on the wiki: http://code.google.com/p/finansu/wiki/Introduction.
You can browse through the source code online, but to download a copy of the whole project you'll need a Mercurial client. I just installed the one called Mercurial 1.8.2 MSI installer and ran
hg clone https://finansu.googlecode.com/hg/ finansu from a command prompt.