What alternatives are there to Fiddler debugging proxy?
NetTool is a developer tool for monitoring and manipulating application-level network messages, particularly useful for debugging web applications and web services. There are two components to NetTool: the HTTP Client, and the TCP Tunnel.1
Paros is for web application security assessment. It is free of charge and completely written in Java. Through Paros's proxy nature, all HTTP and HTTPS data between server and client, including cookies and form fields, can be intercepted and modified.2
1Source:Sourceforge
1Source:Paroproxy
Fiddler is now available for linux and Mac through its Mono port. All instructions available here
I find Burp Suite really handy. It is not Open Source but it is written in Java (works everywhere) and it has community edition. Supports HTTPS, other features are called Essential manual tools