Making a deal with a friendly neighbor is useful, but there are greater benefits in forging a full alliance with another empire.  You may declare an official alliance with at most two other empires.  You may not ally with a third empire until you break one of your existing alliances.   Alliances require a formal declaration of such from both parties, so if one empire fails to declare themselves allied, or if one empire unilaterally breaks an alliance, the other empire will be aware of the situation immediately.
 
Allied empires receive the following benefits:
 

• Allies always share all wormnet portal data with each other.
• Allies may load their ships onto each other's carriers.  (See the Load Ships phase.)
• Allies can never fire guns at one another (except for targeting cargo).  (See the Fire Guns phase.)
• Allied ships count as friendly ships for purposes of determining blockades and interdictions.
• Allies can stockpile RUs at each other's worlds (separate from the world owner's RU stockpile).
• Allies can build ships at each other's worlds (if sufficient allied stockpile is present).
• Allies may repair each other's ships.  (See the Repair Ships phase.)
• Allies always share salvaged ship designs (but not researched ship designs) with each other.
• Allies always share all scan data with each other.
• Allied ships always identify themselves to each other.

 
 
Note that your allies have access to a great deal of information about your empire's map, ship, and scan data, so think twice about forging an alliance with an empire that you do not fully trust.  Some game scenarios require an alliance as a victory condition.