Short answer: everything!!
Collaboration is crucially important in education. From the time that you start any kind of teacher education program, you learn that reflection and collaboration are fabulously helpful in determining effecting lessons. Very often our peers are some of the best resources that are available. And, when those people are not immediately available (if you are the only 5th grade teacher in your building, for example) there are a wealth of resources available online.
Personally, this is one aspect of my teaching experience that I wish were more elaborate. I became the veteran chemistry teacher in my district during my second year of teaching. I was faced with two brand new teachers straight from industry who needed as much help as I did. So, we all fumbled through and survived. These first years together, we worked very closely in order to coordinate the pace and content of our courses, and it was invaluable for all of us. Once we all became more self-sufficient, we all started to look to other resources to learn, and outside workshops became feasible.
These workshops provided the opportunity to network with other teachers as well as the opportunity to bring ideas and materials back to our building. Now, with increasing budgetary issues in education all over the country, outside workshops are becoming more and more difficult to be a part of. However, the Internet provides countless ideas and resources, if you have the time to investigate and filter them.
Basically, if you are able to look past your pride, ask for help, and work collaboratively with other teachers in your building, you will become a better teacher, and your students may have a better learning experience due to the change of pace or new ideas that you try with them.