g; takes you backward one step in the 'changelist'. You can keep g;-ing to go to your last edit, the one before that, the one before that, and so on, and you can use g, to come back forward in the change list. This is similar to how ; and , work with f, F, t, and T.

Also, if you know that you want to go right into insert mode in the place where you last edited (technically, where you last stopped insert mode), you can do gi