Before you start downloading and installing every mod in existence, remember: Though none of these add-ons affect TESO's content, they do affect the way you interface with its world. Many of these are convenient changes for the MMO side of Elder Scrolls Online, and could negatively impact your experience with the game if immersion is your number-one priority. Install at your own risk.
Speaking of installation, here's how to apply add-ons to TESO: First, download the add-on you'd like to use from sites like Curse or ESOUI. Next, extract the add-on folder from the downloaded zipped file. Finally, drop the unzipped folder into the add-on folder in your TESO directory. This is typically located in My Documents > Elder Scrolls Online > live > Addons. Bam, all set! Now, onto the add-ons--and don't forget to expand the images of each slide for a closer look at what's on offer.