I would like to recommend a few really useful (and tested) plugins based on my WordPress experience.
When I searched for a new plugin, the main requirements were:
- Easy to use
- The plugin should only be capable of what is really necessary (due to security reason)
- Small size (due to performance reason)
- Using as little plugin as needed (due to security and performance reasons)
I used these plugins on my WPMU sites for different reasons: