Firefox: The sidebar in Firefox is super customisable, as you can change its position, open stuff like bookmarks or downloads and even load any web page. OmniSidebar makes it easier to use than ever before.
This neat, lightweight extension lets you access bookmarks, history, downloads, add-ons, scratchpad, error console and view page info — all with a handy drop-down at the top to quickly switch between them. The width of the sidebar is customisable and it works just as any of those options would behave in their normal view.
OmniSidebar's real potential is unlocked when you go to its Preferences. You can customise its position, appearance, set a hotkey and a lot more. You can also choose to enable a twin sidebar on the opposite side! I especially liked the ability to set a few things to always open in the sidebar, especially downloads and add-ons manager.