I modeled a character with alot of tentacles, It has a mirror modifier, I was wondering if there was a way to do mirror an armature?
There is the X-Axis Mirror option (EDIT buttons / Armature Panel / Editing Options) when creating an armature, press SH-E when extruding bones.
If you already created an armature, you can duplicate(Shift+D), mirror over X Axis (Ctrl+M) select both and join together (Ctrl+J). But before selecting both and joining together you might want to select the duplicated copy and go into Pose mode and click W, and select “Flip Left-Right Names.” That way when you join, one side has .R bones and the other .L bones.
The only problem is that whichever armature you select first when joining losses the constraints, so you will have to reassign to one side, but at least you got all the bones there!
OR if you don’t have a armature built already you can do what Mike_S said, Shift+E when extruding bones.
Now a trick to mirror vertex groups, remove mirror, duplicate the mesh, mirror over x-axis, rename the groups of one of the sides, change all .R to .L, join the two mesh objects, remove doubles… (the vertex groups will now exist for both sides of the character)