This is particularly handy for common airframes. Once someone has tuned one well, they can upload their configuration file and save others the trouble of having to go through the same process.
I've now created a place in the APM and ArduCopter wikis that you can use to share configuration files. Please post your files in the comments here and I'll upload them to the relevant wikis as they come in.
The APM Configuration File Repository is here.
The AC2 Configuration File Repository is here.
*** Note, 2.0.26, and 2.0.27 had a flaw that switched Alt_hold and Simple mode. Don't fly it. This is fixed in 2.0.28. ***