Here's a list to start you off!

Workshops led by a fellow teacher, central office specialist, principal, consultant

Formal courses or seminars

Residential course, retreat, or institute

Attendance at a professional conference/convention

Sabbatical

Apprenticeship or internship in another organization (e.g., business or industry)

Interschool/interclass visitation

Formal and informal peer coaching; See the Teachers Network Policy Institute paper:

• Peer Coaching for the Improvement of Teaching and Learning by Jean Becker, teacher at Norwood Park Elementary, Chicago (IL)

Teacher centers

Teacher networks; See the Teachers Network Policy Institute papers:

• The Teamin' and Theme-in Network and Collaborative Action Research by Ronald Klemp, teacher at Northridge/Irving Middle School, Los Angeles (CA)
  
• Factors and Processes Necessary to Sustain Networks by Diana Takenaga-Taga, teacher at Windsor Hills Magnet, Los Angeles, (CA)
  
• Strengthen the Model, Nurture the Profession by Peggy Wyns-Madison, teacher at P.S. 131K, New York, (NY)