I've used collaborative whiteboards (boardthing has been a go-to, but trello as well) for virtual or distributed teams with a fair amount of success. Less a retrospect technique, but I also use a survey - like the Happiness Metric, but re-dubbed "Sprint Satisfaction" (not wanting to suggest to my teams "thou shalt be happy or else"). There's always 2 required free-text options for "What would it take to get your satisfaction higher" and "What's one thing you would change".
The survey is sent in advance of retro, and the responses often become discussion points during retro. I use this w/ co-located teams as well, but have found it especially useful in the distributed setting. I've worked with offshore teams where there was more of a deferential attitude where the on-shore team or the team leads were seen as having more of a voice. The survey allows for all team members to contribute.