SeenIt is a Chrome extension that marks the YouTube videos you have already watched so that you can enjoy new content without having to sift through old videos yourself. 

You can also choose to have watched videos removed completely, rather than just marked as watched, via the options menu (click the SeenIt icon in the toolbar).

Get it here!
https://chrome.google.com/webstore/detail/seenit/micbdilkepmleglkjkcgeppifphhpnfl

FEATURES:
-Marks watched videos in homepage recommendations, video search results, and similar videos
-Options menu lets you toggle the removal of watched video thumbnails (click SeenIt icon in toolbar)

COMING SOON:
-Click SeenIt icon in toolbar to view omitted videos
-Whitelist your favorite videos to make them unremovable
-Watched videos that were removed will be replaced with new videos you haven't seen before
-Option to have continuous scrolling on search results
-Manually remove videos you don't like from recommendations
-Option to have this extension predict which videos you don't like and automatically replace them with better recommendations on YouTube