Where are chromium extensions located?

Extensions are stored at ~/.config/chromium/Default/Extensions. Here's an example featuring the amazing adblock:

jake@daedalus:~$ cd .config/chromium/Default/Extensions/
jake@daedalus:~/.config/chromium/Default/Extensions$ ls
gighmmpiobklfepjocnamgkkbiglidom/  jggheggpdocamneaacmfoipeehedigia/
jake@daedalus:~/.config/chromium/Default/Extensions$ cd gighmmpiobklfepjocnamgkkbiglidom/2.3.1_0/
jake@daedalus:~/.config/chromium/Default/Extensions/gighmmpiobklfepjocnamgkkbiglidom/2.3.1_0$ ls
adblock.js        background.html  blacklisting/         button/     functions.js  img/        jquery/    manifest.json  pages/   whitelister.js
adblock_start.js  blacklister.js   broadcast_channel.js  filtering/  Icon.png      Info.plist  _locales/  options/       port.js

To see it in your file browser, make sure you can see hidden files (View > Show Hidden Files). Or, do like Gaurav says and type in the path after pressing ctl+l.

The other answers are wrong if you are not using the default profile (e.g. have multiple users or specified the user profile path).

Visit chrome://version/ in your Chrome and you'll see the "Profile Path".

Type this in Nautilus after pressing "ctrl+L"

~/.config/chromium/Default/Extensions