Vote for your favorite extension

[Cqoicebordel]

Toogles

[Hadden89]

Bulk Download Images (ZIG)

[TbGbe]

ImTranslator

[Zakro]

ContentBlockHelper

[ROTFL]

Eliminator Slajdów

[Stardust]

YouTube Plus (by Particle)

https://chrome.google.com/webstore/detail/youtube-plus/nkdalpbojfdilmiboaiedicdbigdabpb
also available for Firefox

[Ayespy]

The very boring Auto Copy which I use not because it auto-copies, but because it permits plain-text copy.

[Ayespy]

XMarks

[U.F.U.K]

Disconnect

[duarte.framos]

A TabMixPlus-alike

[duarte.framos]

Downloads Router (doesn't currently work in Vivaldi as far as I know, unfortunately)

[duarte.framos]

Image Viewer

[ugly]

FacebookBlocker

[ugly]

Facebook Redirect Fixer

[duarte.framos]

Scrollbar Anywhere
For me without this middle-mouse scrolling is utterly unusable

[pafflick]

Imagus

Shows large images (if available) when hovering over the small thumbnail.

[pafflick]

Viewhance

Adds features for the browser's default media viewer.

[U.F.U.K]

HTTPS Everywhere

[pafflick]

Google Translate

[pafflick]

Custom Style Script

It's not too handy for editing the code, but it allows injecting custom JavaScript and CSS on specified (or all) websites. I haven't found a better extension for this purpose (so far).