Alternatives to His-tags
This depends on the technology you're using for the interaction detection.  Surface Plasmon Resonance and Isothermal Titration Calorimetry (and others) don't require tagged proteins of any kind.  Though it doesn't hurt either one.

If you're looking purely at chromotography, any tag that will stably capture. I commonly see GST, FLAG, and MBP in order of occurrence.