A simple mod that lets you pair sets of signs together to allow teleporting between them. This mod works both client-side or server-side, and if running on a server, clients don't need the mod installed.
Simply right-click any two signs with the "pairing item" (a diamond by default, configurable) to pair them together. The item will be used up when right-clicking the second sign to finish pairing.
Breaking either sign of a pair will un-pair them and give back the diamond.
On first run, a config file is created at <minecraft_dir>/config/warpSigns.json. This can be edited to change the pairing item, whether non-admins can pair/edit signs, etc.