Which angles do you want?
Which camera mode are you using?
There's a
viewtaflip command that can
be bound (ex: bind c+u viewtaflip) that will
flip the TA camera mode by 180 degrees
(technically a spin, but whatever

). A custom
widget could easily be setup to setup other
angles for camera modes like Free Camera,
FPS, TW, or Rotate Overhead.
P.S. Try the Free Camera with the Smooth Move
widget enabled. Holding the space bar with the
middel mouse button allows you to change your
viewing angle easily, and you can move around
by just using the middle mouse button.