Musical Works and Performing Rights Organizations
BMI was founded in 1939 and is now the largest PRO. You can search BMI's database of over 8.5 million songs here.
SESAC is much smaller, with around 400,000 songs, but it has many big name artists and boasts that it pays its royalties out monthly instead of quarterly. You can search SESAC's catalog here.
A fourth PRO, Global Media Rights (GMR), was founded in 2013 and already has many major artists in its catalog, ranging from Bruno Mars and Drake to John Lennon and Billy Idol.