The IT Stuff

Ethereal
TcpDump/WinDump
Snort
Microsoft Network Monitor
eEye Iris
TamoSoft CommView
WildPackets EtherPeek
Network Associates Netasyst

Ethereal® is used by network professionals around the world for troubleshooting, analysis, software and protocol development, and education. It has all of the standard features  you would expect in a protocol analyzer, and several features not seen in any other product. Its open source license allows talented experts [...]

A Sniffer is a program or a device that eavesdrops on the network traffic by grabbing information travelling over a network. Sniffers basically are “Data Interception” technology.
OR
is a program that monitors and analyzes network traffic, detecting bottlenecks and problems. Using this information, a network manager can keep traffic flowing efficiently.
Shared Ethernet:In a shared Ethernet environment, [...]

Monitor networks boundaries for attacks.
Ensure that routers are not converting layer 3 broadcasts into layer 2 broadcasts. The Cisco command to disable this is: no ip directed-broadcast. This is the default setting for routers that use IOS version 12.0 or greater.
Restrict routers to allow only the use of ports that are necessary for the site [...]

Can not protect against the inside attacks
Firewalls may represent a significant bottleneck in communication between the protected network and the outside world.
Very little or no effort is taken to look in detail at the data contents of the packet.
Firewalls do not protect against viruses: New viruses are continuously released and there are many ways of [...]

HTTP:

HTTP  Code Red, Nimda Worms & Mutations
SQL Injection
Directory Traversal Attacks
MDAC Buffer Overflows
Cross-Site Scripting Attacks
Chunked Transfer Encoding Attacks

SMTP :

SMTP Worm
MIME Attacks
SPAM Attack
Command Verification Attack
SMTP Error Denial-of-Service Attack
Mailbox Denial-of-Service Attack (excessive email size)
SMTP Mail Flooding
Address Spoofing
SMTP Buffer Overflow Attacks

FTP:

FTP Bounce Attack
FTP Port Injection Attacks
Directory Traversal [...]