The ideal collaboration between students looks a lot like discussion. Guided discussion, granted, but discussion. Students who collaborate share the knowledge they have in order to gain even more knowledge, especially if the students are grouped in ways that benefit each other. If you only put people together who are at the same level, they don't necessarily push each other to higher levels.
In my sign language class, the students share knowledge as they practice their signs. Some are better at making the signs than the others, but when they practice together, they both get better. Sign Language is all about participation. It's not just about what they learn from me, but it is about what they learn from each other, too. Just today one of the students told me that she liked the conversations she was having with the others because she was learning how to sign and read the signs better.
In my creative writing class the students share their poetry with each other to get feedback and suggestions. Later this semester, when they are writing prose, we have a couple of assignments where the students actually collaborate on their writing by doing a collaborative writing assignment. The students plan a story and write it together on a Google Doc so that each of them sees what the other is doing as he or she is doing it.
In my English classes we use a site called CollaborizeClassroom that allows the students to put forth their ideas about a topic onto a forum for discussion. The students who would normally not talk out in class are more willing to put forth their ideas here.
The students who can learn how to work collaboratively will later be better equipped for the workplace and other real world situations they will inevitably find themselves in.