Desai Abhishek: Packet analyzer

A packet analyzer (also known as a network analyzer, protocol analyzer or packet sniffer, or for particular types of networks, an Ethernet sniffer or wireless sniffer) is a computer program or a piece of computer hardware that can intercept and log traffic passing over a digital network or part of a network.

Uses of packet analyzer:

Analyze network problems
Detect network intrusion attempts
Detect network misuse by internal and external users
Documenting regulatory compliance through logging all perimeter and endpoint traffic
Gain information for effecting a network intrusion
Isolate exploited systems
Monitor WAN bandwidth utilization
Monitor network usage (including internal and external users and systems)
Monitor data-in-motion
Monitor WAN and endpoint security status
Gather and report network statistics
Filter suspect content from network traffic
Serve as primary data source for day-to-day network monitoring and management
Spy on other network users and collect sensitive information such as login details or users cookies (depending on any content encryption methods that may be in use)
Reverse engineer proprietary protocols used over the network
Debug client/server communications
Debug network protocol implementations
Verify adds, moves and changes
Verify internal control system effectiveness (firewalls, access control, Web filter, spam filter, proxy)

A list of few sniffers available for download from the Internet:
http://sectools.org/tag/sniffers/

Screenshots of major packet sniffer applications:

1) Wireshark: