Continued from Part 1...
NAME: Detonator
DESCRIPTION: Allows you to place proximity mines which, when tripped, cause big splash damage. Secondary fire simply deactivates mines you've already placed so that you can collect them again.
USEFULNESS: Tremendously useful, but only in specific circumstances. When defending a specific point, for example, the detonator can guard entrances without you having to worry about them. Basically this is a must-have for battles where you can prepare before the onslaught begins.
NAME: Contact Beam
DESCRIPTION: Fires a single, highly-damaging, charged shot at an enemy. Useful against large, slow enemies. Secondary fire causes nearby enemies to be knocked off their feet.
USEFULNESS: Not very. It's rare you're only fighting a single, large enemy, and even in the cases where you are, the secondary fire from the line gun is plenty effective.
NAME: Force Gun
DESCRIPTION: Basically a shotgun. The primary fire on the force gun emits a close-range blast of kinetic energy, sending enemies back and oftentimes blowing limbs off. Secondary fire is the same idea, but at longer ranges. The spread is much more narrow, though.
USEFULNESS: Arguably the best close-range gun in the game, the force gun is good option when you're surrounded by enemies. At range, though, it's pretty useless, as the secondary fire attack does very little damage.
NAME: Flamethrower
DESCRIPTION: It's, well, a flamethrower. Primary fire shoots out a stream of flames, while secondary fire launches an incendiary grenade.
USEFULNESS: Another good pick for close-range, as it can stun just about any enemy, and make short work of crowds of smaller enemies. Just be wary of the long reload time, as you can easily find yourself overwhelmed by killer babies.
NAME: Ripper
DESCRIPTION: Primary fire causes a spinning blade to hover in front of Isaac, which you can then maneuver into enemy limbs for a few seconds. Secondary fire shoots a blade like a projectile.
USEFULNESS: Like the flamethrower, the ripper can make short work of smaller enemies in your vicinity, and can take down one or two larger enemies if you're quick with your blade movement. Definitely takes some getting used to, though.
NAME: Seeker Rifle
DESCRIPTION: It's a sniper rifle, pure and simple. While it's not very good at cutting limbs, it deals tremendous damage to anything it hits. Alternate fire allows you to zoom in on a target, dealing even more damage.
USEFULNESS: For single enemies at long range, there's no better weapon. It's awful at dealing with more than one enemy at a time, though, thanks to a small clip size and long reload time.