Linux is a clone of the Unix kernel, written from scratch by Linus Torvalds with assistance from a loosely-knit team of hackers across the Net. It aims towards POSIX and Single UNIX Specification compliance. It has all the features you would expect in a modern fully-fledged Unix kernel, including true multitasking, virtual memory, shared libraries, demand loading, shared copy-on-write executables, proper memory management, and TCP/IP networking.
fio is an I/O tool meant to be used both for benchmark and stress/hardware verification. It has support for 13 different types of I/O engines (sync, mmap, libaio, posixaio, SG v3, splice, null, network, syslet, guasi, solarisaio, and more), I/O priorities (for newer Linux kernels), rate I/O, forked or threaded jobs, and much more. It can work on block devices as well as files. fio accepts job descriptions in a simple-to-understand text format. Several example job files are included. fio displays all sorts of I/O performance information, including complete IO latencies and percentiles. Fio is in wide use in many places, for both benchmarking, QA, and verification purposes. It supports Linux, FreeBSD, NetBSD, OS X, OpenSolaris, AIX, HP-UX, and Windows.
Snort is a network intrusion detection and prevention system. It is the most widely deployed technology of its kind in the world. It performs detection using a variety of methods including rules-based detection, anomaly detection, and heuristic analysis of network traffic. Its rules language is open source and available to the public as well.
libdvdcss is a cross-platform library for transparent DVD device access with on-the-fly CSS decryption. It currently runs under Linux, FreeBSD, NetBSD, OpenBSD, BSD/OS, Solaris, BeOS, Win95/Win98, Win2k/WinXP, MacOS X, HP-UX, QNX, and OS/2. It is used by libdvdread and most DVD players such as VLC because of its portability and because, unlike similar libraries, it does not require your DVD drive to be region locked.
Rsyslog is an enhanced multi-threaded syslogd. Among others, it offers support for on-demand disk buffering, reliable syslog over TCP, SSL, TLS, and RELP, writing to databases (MySQL, PostgreSQL, Oracle, and many more), email alerting, fully configurable output formats (including high-precision timestamps), the ability to filter on any part of the syslog message, on-the-wire message compression, and the ability to convert text files to syslog. It is a drop-in replacement for stock syslogd and able to work with the same configuration file syntax.
iptables is built on top of netfilter, the packet alteration framework for Linux 2.4.x and 2.6.x. It is a major rewrite of its predecessor ipchains, and is used to control packet filtering, Network Address Translation (masquerading, portforwarding, transparent proxying), and special effects such as packet mangling.
NoMachine NX is a fast terminal server and desktop virtualization system based on the X11 protocol. NX is an order of magnitude faster than VNC or X11 and can run on bandwidth as narrow as 10 kBit/sec. In addition, NX translates and embeds the MS Windows Terminal Server and VNC protocols into X/NX, enabling users to compress and accelerate remote Windows and VNC sessions. The NX project provides a suite of libraries and X11 proxying agents implementing efficient compression and optimized transport of X11, SMB, IPP, HTTP, and arbitrary protocols like audio and video over the Internet.