Does anyone know of a solution that provides functionality to Microsoft Sharepoint that offers Stackoverflow like functionality?
I would really recommend not using SharePoint for this kind of feature. SharePoint is great at what it does (document management, collaboration, etc) but absolutely terrible at everything else.
Are you forced to develop for SharePoint, or can you just do it in ASP.NET/C#?
Any SharePoint functionality that did with would most likely be completely custom ASP.NET pages that would run just as well as a stand-alone product.
Rather than looking for a SharePoint-specific product, look for one that works well with the general microsoft stack, and you should be able to integrate it fairly well with whatever SharePoint site you're running.
You could look at using StackExchange (either hosted or local for a price!) when it becomes available in September(ish), but that is probably overkill.
There is the Idea Center feature in Newsgator Social Sites, check the screenshot. I haven't used this although our company is trialling this product so I hope to have a look at it soon.
The downsides are that this is part of a much bigger (albeit pretty cool) product and isn't free.