Sometimes I have quite a bit of trouble finding bombs or I will need a bomb in order to get more bombs. What ways can I break rocks (via items or enemies) if I have zero actual bombs?
The following activated items can destroy rocks:
Anarchist Cookbook - Spawns 6 bombs in random locations so there's not a 100% chance that it'll break the rock you need.
Bobs Rotten Head - Lets you throw a bomb in any of the 4 cardinal directions.
Dead Sea Scrolls - Activates as any other random activated item, so it's super unreliable.
The Doctors Remote - Activates a crosshair that will drop a bomb on that location (which can be changed) after a designated amount of time.
Kamikaze! - Acts as if you blew up a bomb on your location.
Mr. Boom - Drops a big bomb on your location.
The Nail - Allows you to walk through rocks for the room you activated it in, and instead of flying over them, you destroy them.
Additionally, one passive item, Fetus in a Jar, allows you to shoot bombs as tears, and one pill, Explosive Diarrhea, causes you to drop a few bombs as you walk.
Also, if you can kill a Mulligoon near a rock, or prompt him to suicide near one, he'll drop a bomb on his death. Boom Fly's also serve the same purpose, except they immediately explode instead of dropping a bomb. Fred fires projectiles in an upwards arc that explode upon hitting the floor, but they don't have much range so you need to wait for him to get close enough to the rock you want to destroy, stand near it, and dodge his projectile matador style.
You can also use Troll Bombs (which drop in place of regular bombs sometimes) dropped from activated items and tarot cards, but in those situations, without the troll bomb you'll get a regular bomb anyway, so they're not very practical.
Another way to break rocks is to use the High Priestess Tarot Card to call down mom's foot on a random enemy in the room. The foot will break any rocks around the impact point with about the same radius as a bomb explosion. This strategy is rather impractical though, as it occurs on a random enemy (can be eliminated by having only one enemy left) and you have to get the enemy to the rock you want to destroy (which can be rather hard with some enemies).
The Wrath of the Lamb expansion also adds the Notched Pickaxe, another activated item which allows you to break one rock per recharge (one room) as well as the passive item IPECEC which behaves similarly to the Fetus in a Jar pickup, only the projectiles fly through the air and explode on impact.
In addition, Wrath of the Lamb also adds many new enemies with explosive attacks (only applicable in the new floors). These can be used in the same way as Fred projectiles, by waiting near a rock and dodging the attack when it comes.