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Like the Nudging tool of Photoshop moving pixel by pixel an element. Well It's there for you in XSI.
This tools works only on marked parameters and will only work on the first one marked, for instance if you mark pos it will only work on posx.
Now keep in mind that you can use it for virtually any parameters that are markable in XSI. It's being used in conjuction with transform -> transformation setup -> translation. The value in this field will allows you to determine by how much you want to nudge.
Usage : Select a 3D Object. Mark a parameter like posx. Run the script (by default if you haven't modified the translation in transformation setup you should move by 1 in positive X)
Now you want to nudge negatively, simply press and hold ctrl while running the script.
Note that you could attach that script to keypad and just nudge pressing the ctrl up and ctrl down arrow.
This is good for 1.5 and up
Have fun nudging away.