Generic Routing Encapsulation
A very useful networking standard defined in RFC-1701, -1702 and -2784, GRE can be used to do all kinds of "stupid network tricks" as well as implement several categories of encryption and network security. In its most basic form, GRE allows any network-layer protocol (or in some cases, protocols from other layers, e.g. Ethernet frames) to be encapsulated in any other network-layer protocol. This means that IPX packets can be injected into an IP network and dissected at "the other end" of a connection -- allowing two IPX networks to be joined over a public IP network such as the Internet or over a private enterprise-wide network running IP. Also, GRE allows for IP-in-IP encapsulation which makes it easier for networks to transition to IPv6 networking in a piecemeal fashion and also lets certain types of IPsec packets traverse a traditional IP network that would otherwise remove these security features (or vice versa).
In its current form, GRE has been implemented in most UNIX network stacks, Cisco routers and other pieces of network equipment. Since these devices all (mostly) adhere to the RFC standards, they all can pass encapsulated network traffic between each other and any of them can be used to encapsulate or decapsulate the internal protocol.
GRE is closely related to (in function and design) PPTP, although not as specific.