RL does have a solo album. It's titled RL: ements. i listened through it a few times and really wasn't feeling it, but it's worth a listen.
I don't really listen to too many "groups" so I have no one that I want to hear go solo. Beyonce would be at the top of my list...if she hadn't already gone solo
Michael Jackson co-wrote the single "We Are the World" which was made to aid people in Africa and sold nearly 20 million copies, one of the best-selling singles of all time.