During the years prior to the Battle of Yavin (the early rebellion) there were a number of different groups that opposed the Empire. Many of these said groups would eventually join together to become the Rebel Alliance (and to a lesser, more ideologically extreme degree, the Partisans.) But while looking at the main pages of these rebel organizations, it came to my attention that surely there would have been other smaller opposition groups that either were not as successful as the Partisans and Rebel Alliance, or were crushed by the Empire.
So, were there any groups that opposed the Empire and that were not part of a major rebel organization, such as the Rebel Alliance or the Partisans?
I'll take legends or canon answers.