As of patch 1.21, base jump pads are now only usable by the defending team (i.e. red team on red side).
Jump Pad location by map:
Note: This doesn't take base jump pads into consideration, since every map has them.
This map has a single jump pad in the middle of the bottom lane, which will take 'nauts who use it all the way up into the top lane, going through the jungle in the process.
This map has 4 jump pads: two facing opposite directions in the middle of the upper portion of the top lane, which take 'nauts who use them up to the creep areas, while the other two are in front of the outer turrets of the bottom lane, taking 'nauts to the top lane when used.
This map has 2 jump pads, each behind the outer turrets. Only accessible to teams of the opposite side after the turret's been destroyed. These can take players to the top lane right in front of its inner turret, where the creep releaser is or, with player adjustment, to the side entrance of the creep area.
This map has two jump pads above the bottom lane. These can be accessed from the bottom lane itself or by dropping down the glass platforms immediatly above them. They take players to moving glass platforms above them, although an extra mid-air jump will allow some 'nauts to get to the top lane directly.