The event(force:navigateToComponent) can be found in app.js which is used in Salesforce1 internally. However no docs introduce it until now(Spring '15).
Definition details,
{
"$descriptor$": {
"prefix": "markup",
"$namespace$": "force",
"name": "navigateToComponent",
"$qualifiedName$": "markup://force:navigateToComponent"
},
"$superDef$": {
"$descriptor$": {
"prefix": "markup",
"$namespace$": "aura",
"name": "applicationEvent",
"$qualifiedName$": "markup://aura:applicationEvent"
},
"$superDef$": null,
"$attributeDefs$": { },
"type": "APPLICATION"
},
"$attributeDefs$": {
"componentDef": {
"name": "componentDef",
"type": "aura://String"
},
"componentAttributes": {
"name": "componentAttributes",
"type": "aura://Object"
},
"forceServerDependencies": {
"name": "forceServerDependencies",
"type": "aura://Boolean"
},
"resetHistory": {
"name": "resetHistory",
"type": "aura://Boolean",
"default": false
},
"ignoreExistingAction": {
"name": "ignoreExistingAction",
"type": "aura://Boolean",
"default": false
}
},
"type": "APPLICATION"
}