Real time symmetry modeling In maya there a way to move vertices or whatever in symmetry without using a modifier. That is usefull in making changes after a model with its uvs for example are already made and you want to tweek things a bit. But using the mirror modifier to do such thing will destroy the uvs of half the model.
Its also pretty fast and handy.

Is there a way to do that in blender?
You want to use X-Mirror. It only works on the X-axis but doesn't require a mirror modifier. While transforming and other simple operation work well, more complex operations like deleting or disolving may not be mirrored correctly.

From this answer:


  Sounds like you [want] the X-mirror option.
  
  Open up the tool shelf T switch to the Option tab and
  [check] the X Mirror checkbox.