If you found money lying on the ground would you pick it up and take it even if you didn't need it or leave it for someone else to find?

You lost me at "if I didn't need it." Irrational premise.

Don't like litter. Public spirited thing is to pick it up.

I was walking with my son and noticed a ten dollar bill lying on the sidewalk. I told him to see what that object was and he picked it up. 

If it was just paper money without a wallet/purse, and no one was around, I would pick it up.  It's part of the cosmic cycle of lost & found.  I've lost money in that way myself.  So, I just look at it as the universe returning what's owed to me.  

have found money before
Dollar bills blowing all over the street
always a need somewhere for it

I would pick it up.  My next steps would be determined by where I found the money and the amount.