Alt+Connect manages dialup connections, allowing a group of networked machines to share a single Internet connection through their server. Custom client software lets users to start or stop their internet connection, and the server (connectd) ensures that the connection remains up only while someone's using it. Features include support for multiple ISPs and links (modems or ISDN channels), connection control lists, ability to record the time a user spends online and charge him for it, and the ability to enable and disable IP forwarding as a machine starts or stops using the Internet.
DeleGate is a multi-purpose application level gateway or proxy server that mediates communication of various protocols, applying cache and conversion for mediated data, controlling access from clients, and routing toward servers. It translates protocols between clients and servers, converting between IPv4 and IPv6, applying SSL (TLS) to arbitrary protocols, merging several servers into a single server view with aliasing and filtering. It can be used as a simple origin server for some protocols (HTTP, FTP, and NNTP).
Nmap ("Network Mapper") is a utility for network exploration, administration, and security auditing. It uses IP packets in novel ways to determine which hosts are available online (host discovery), which TCP/UDP ports are open (port scanning), and what applications and services are listening on each port (version detection). It can also identify remote host OS and device types via TCP/IP fingerprinting. Nmap offers flexible target and port specifications, decoy/stealth scanning for firewall and IDS evasion, and highly optimized timing algorithms for fast scanning.
tcpxd is a TCP/IP relay or proxy, allowing a connection to a port on a system to be forwarded to another port on any other system. It is useful for firewalls and service relocations, and is small, simple, and fast. It's also easier to learn than netcat and supports TCP_NODELAY, alternate local ports, partial connection closures, and non-blocking connects.
Harm acts as a four-way socket redirector that allows you to effectively make a TCP connection from the Internet to a host behind a Linux masquerade-style firewall. The server (behind the firewall) makes connections to the client (on the Internet). After a successful connection, it will bounce packets from a telnet client (Windows and Linux) to the Harm client, to the Harm server (Linux only), or to the telnet daemon behind the firewall.
Actim allows you to send and retrieve files, and to call command line programs on a target machine via emails. The application can handle both synchronous and asynchronous requests with basic security features. It has both command line and graphical user interfaces. This can be useful if, for example, your home PC does not have any access to your office workstation which is behind the company firewall.
Proxytunnel is a program that connects stdin and stdout to a server somewhere in the Internet through an industry standard HTTPS proxy. It's mostly used as a backend for OpenSSH's ProxyCommand, and as a proxy backend for Putty. It can also be used for other proxy-traversing purposes.