We use Apple's Cocoa framework, often coupled with PyObjC to make some of our applications.
Tutorials We Like
Cocoa Dev Central's How To Support Applescript
Interesting Objective-C classes
AMSerialPort, AMShellWrapper (serial port, shell commands)
SerialPort.framework (commercial, $50) (serial port)
ConnectionKit - connect to TTP, FTP, SFTP, WebDAV, .Mac and to Amazon S3 sites
Interesting Sample Code