Organizational development — looks at the company's communications, shared values and beliefs, common behaviors, and unwritten rules, as well as management styles, involvement, decision-making, and teamwork. Though good consultants can come from many backgrounds, look for a master's degree (or a doctorate) in organizational, social, or I/O psychology - since organizational development is applied psychology.
Survey — a specialist in getting information from customers and employees using surveys. This field is filled with people who talk a good game but produce poor surveys. Their training ranges from a single class (if that) to years of training and supervised research. Some consultants have years of experience in selling worthless surveys to big-name corporations. It's hard to find the right survey expert, but if they focus more on the process and follow-through - in turning data into action - it's a good sign.
Human resources — looks at compensation, benefits, liability, sexual harassment, discrimination, and other issues; may overlap with organizational development. Some people are specifically trained in human resources, while others have just a course or two; a good "HR" consultant will be up to date on legal issues as well.
Compensation and feedback — two very different skill sets are needed for compensation and feedback consultants. A specialized field, compensation may involve extensive knowledge of legal issues, comparative compensation surveys, and other such financial knowledge; but feedback is used largely for development, and relating 360°, peer, or subordinate feedback to compensation can be disastrous, and most compensation consultants, in our experience, do not have the needed knowledge for development-oriented feedback (and vice versa).
Finance — looks at financial systems, accounting, business ratios, mergers and acquisitions, sometimes marketing. May handle organizational development, but often has little training in these areas compared with dedicated professionals; it is easy to fall into the trap of “it looks easy enough.” An MBA is de rigeur.
Marketing — looks at marketing issues and, often, market research. Often, different people handle the marketing and market research ends, because different skills are needed. The best market researchers do not rely solely on one method (e.g. paper surveys or focus groups).
Project management — expertise in the process of managing large projects can be tapped or transferred to the organization by using a project management consultant. The best of these can break down massive projects into digestible bites that people can easily deal with, while keeping the overall goal in mind and everything on track; the worst simply generate charts on Visio and find out who to blame if things fall behind. There is a definite need for project management skills, and a dedicated consultant with training or experience in the field may be invaluable.