I recently released a technical analysis library called Tulip Indicators under the LGPL license. If you need to do technical analysis, I think it's the best library out there now.
I also released a companion project called Tulip Cell which provides the functions to Excel spreadsheets via an add-in.
And finally, I'm working on a full featured charting program that I plan to release soon: Tulip Charts.
The Incomplete Beta function, or regularized incomplete beta function, seems to be one of those functions that should be built into the standard libraries, but is conspicuously missing. It shows up all over the place in statistics. It's crucial to Student's t distribution, which is likely the second most important distribution in statistics after the Gaussian distribution.
I couldn't find an open-source implementation to fit my exact needs, so I'm going to give simple C code to compute it.
If you're just looking for code, check out the Github repo. If you want to learn how it works, keep reading.
Several open-source implementations of the incomplete beta function already exist, but none of them were going to work for my project.