Sometimes the simplest of scripts in button form become the best things ever. I wrote this in two minutes, committed it to a button on the tool shelf and it's been indispensable since. It's function is simple, take two points (ie. vertexes,joints,groups, you name it!) and mirror them across the X axis. It's useful for fixing things like vertexes on a rig that somehow got un-mirrored in the modelling process, or joints that you thought were aligned, no longer are. It's an answer for some of those problems where you're wondering, if copy attributes or mirror attributes are enough. I find myself more times just wanting to mirror the thing I wanted, rather than dig through panels and windows. 

"""
Positive X mirror
"""
s = cmds.ls(sl = 1,fl=1)
if s.__len__() > 2:
    raise ValueError("More than 2 objects selected, expected only 2")
elif s.__len__() <= 1:
    raise ValueError("Not enough objects selected, expected 2")
else:
    pos = cmds.xform(s[0],q=1,t=1,ws=1)
    negX = pos[0] * -1 
    newPos = [negX,pos[1],pos[2]]
    cmds.xform(s[1], t = newPos,ws=1)

And the opposite...

"""
Negative X mirror
"""
s = cmds.ls(sl = 1,fl=1)
if s.__len__() > 2:
    raise ValueError("More than 2 objects selected, expected only 2")
elif s.__len__() <= 1:
    raise ValueError("Not enough objects selected, expected 2")
else:
    pos = cmds.xform(s[1],q=1,t=1,ws=1)
    negX = pos[0] * -1 
    newPos = [negX,pos[1],pos[2]]
    cmds.xform(s[0], t = newPos,ws=1)

It's premise is stupidly simple: take one world-space translation values, and mirror those on the second object selected and make a  x * -1 calculation, while straight copying the other values. The first selected object is indexed at [0] and the second at [1], just so we're clear. Also, I edited it to make an opposite version.

Note: This currently only supports two selected objects. I do plan on extrapolating this out to maybe a GUI-based mirroring setup, or maybe use more intelligent math to determine the appropriate distances.  

Word of warning, I wrote this in Python using only commands Maya supports in their documentation, I use Maya 2011, but I don't think there's anything that would've been eliminated or changed to make this break. Test it out, see if it works for you, and if it does, then great!

Update: Forgot to include the fl=1 flag in the original code.