EffeTech HTTP Sniffer is a windows binary that captures and analyzes HTTP traffic.
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Ace Password Sniffer is a windows executable that captures usernames and passwords sent through http, ftp, smtp, pop3, telnet.
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Syscall Tracker is a very powerful tool for Linux 2.2 and 2.4 which allows you to write rules to track system calls. It includes a kernel module plus a userspace applications. Currently only logging the invocation is supported, but in the future, you will be able to fail the system call (i.e. force it to return some error code), or suspend the process executing it. Allows you find out info that is hard to find, for instance to determine which process touched a certain file.
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Nessus is a free, up-to-date, and full featured remote security scanner for Linux, BSD, Solaris and some other systems. It is multithreaded, plugin-based, has a nice GTK interface, and currently performs over 920 remote security checks. It has powerful reporting capabilities (HTML, LaTeX, ASCII text) and not only points out problems, but suggests a solution for each of them. Windows version available here.
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