What are the lumps (Circled) on top of these two Boeing 737s? I looked it up but I only found the answer for one specific Virgin Blue 737.
Original image source
It is the antenna for the inflight wifi system. The Southwest aircraft has a Ku band antenna on the top, allowing it to access the internet through satellites. The United aircraft appears to have a similar system.
It looks like satellite data link antenna discussed here. It provides internet connection for passengers.