From b2evolution manual

This plugin adds a widget that displays a list of recent comments in your sidebar.

[edit] Download

SidebarComments plugin at SourceForge

[edit] Installation

1. Copy the sidebarcomments_plugin folder to your plugins directory.
2. Go to Global Settings >> Plugins Install >> Install New and click 'Install' next to the plugin in the list.
3. Go to Blog Settings, select the blog you want to add the widget to, the click the Widgets tab.
4. Find the container you want to add it to (usually Sidebar) and click Add Widget.
5. Under the Plugins heading, click on Sidebar Comments.
6. Finally, you can reorder the widgets to position the new one where you want it.