Frontends are made to simplify or enhance Mame. They often have better support for additional media and allow better access to the huge amount of data Mame stores about it's systems and software. Here are some of the more popular...
QMC2 stands for Quick Mame Catalog / Launcher II. This is one of the more popular frontends for Mame. It is always up to date and is very customizable. It is also available on multiple operating systems. You can get it from here.
Emuloader is a powerful frontend that support pretty much all the available media available for Mame. It is always up to date and there is a support forum available as well. Emuloader also supports other emulators as well. You can get it from here.
GameEx is the perfect frontend for a cabinet. It has great support for controllers and is incredible customizable. You can get it from here.
Hyperspin, much like GameEx is the perfect frontend for a cabinet. It also has great support for controllers and brilliant user interface. You can get it from here.