CRC32 check for .NET
You know, I remember I time when it was uncommon to find .NET implementations of useful code.
CRC, Cyclic Redundancy Check (and not Class, Responsibility, Collaboration, see XP) is useful for discovering whether changes have been made to a collection of bytes. CRC creates a signature for a collection of bytes, and there's a very low chance that two collections will have the same CRC signature. (This is true for even minor changes to a file.)
The guys at vbAccelerator have a CRC library for .NET, if anyone else out there needs one of these today. Check out the rest of their libraries for more cool bits and pieces.