What if any links exist to free Excel "helper" class libraries for C#?

In terms of Office automation for Excel, there are some good PowerShell Open XML cmdlets that can be used for PS or ported to C# at Announcing the Release of PowerTools for Open XML V1.1. Eric White's blog, where this link is, has recently done an more in depth look at Excel automation using Open XML, like Table Markup in Open XML SpreadsheetML. Note about Open XML - although it is the preferred method by many, you don't actually have to use the Open XML SDK - you can just use System.IO.Packaging to gain access to Excel 2007/2010 files.

Also, often overlooked, but the Excel snippets available for VSTO, Open XML and Interop development are great. 1, 2, 3

Also, check out the Excel samples for VB @ http://msdn.microsoft.com/en-us/library/8x19fbw1(v=VS.90).aspx. These come installed with VS when you also choose VB as a language during the install.

Update: Just discovered the All-In-One Code Framework (Office) which has a lot of great Excel helper classes. You can find it on: http://1code.codeplex.com/releases/view/51868.

VSTO Power Tools is a good set of utilities to work with Excel:
http://msdn.microsoft.com/en-us/magazine/dd263100.aspx

ExcelDNA is a very cool project. It provides a lightweight way to consume .NET code from Excel, with a deployment model which is much easier than VSTO. I found it an interesting alternative to VSTO if you want to write UDF in .NET for instance.