is this some type of sponge?

[sfgiantfan]

This black thing is huge. It is about 5" long. Is it harmful and if it isn't, do people usually keep these in their tanks or get rid of them?

 

[ccCapt]

It looks to be a sponge and more than likely harmless. You might be able to find something similar here.
Sponges

 

[sfgiantfan]

Thank you. I will check it out.