Well there are several different developers who make a couple different types of custom firmware.
There is DEX firmware & CEX firmware. CEX firmware is the most common, and the most common ones you may have heard of are Rogero & Habib. CEX is considered the 'basic' custom firmware. DEX is less common, but used by many people none-the-less. DEX firmware is installed by changing your consoles flash type to accept 'debugging' firmware, which can be pretty difficult for beginners, hence most people stick with CEX. Examples of the most common types of DEX firmware would be Rebug or PS3ita. Though, Rebug also make CEX firmwares.
As for there being a main one that everybody uses, there isn't. Its very much personal choice. Each developer adds their own little features & people decide which one they like best. My personal favorites are Rogero for CEX, and PS3ita for DEX.
Message me if you have any other questions.