Usability Boost for Google Plus improves readability on Google+ and adds some useful features.
Design:
– small CSS changes that put the focus on the content, separating visually posts and comments, improving design on the profile page.
– post infos showing up on rollover only

Features:
– The notification bar stays at a fixed position on top so you’ll never miss a notification again !
– Displays a direct link to mute post (very handy)
– Ability to star a post so that you can read it later !

Google+ Ultimate for Google Plus. This is an extension for Google Plus to add functionatility and themeing for the site.

Features
-Adds stylized theme to Google Plus to add to the experience and make it easier to read.
-Adds notifications to the extension icon (beta)
-Enables floating top and left navigation along with an ultra compact navigation to help minimalize the navigation
-Enables an option side-by-side status viewing and/or scrollable statuses(off by default)
-Allows option to move Chatbox to the top navigation as a drop down button. (on by default, but can be turned off)
-Thin scrollbars option (on by default)
-Resize images and embeds to a smaller thumbnail (for those who prefer Facebook’s style of image sharing)