We don't do a "Volunteer Open House" but we do have a volunteer social every other month.  Current volunteers are encouraged to bring a potential volunteer as a guest.  We've gotten several volunteers this way. 

I haven’t done an open house per say but I’ve hosted volunteer teas just to bring potential volunteers in, show them around, tell them about our program, etc.  ours was very informal we just put out flyers and personal invitations I think I ran something in the local paper. We had program information available and current volunteers there to share.