As a complement/update for Loki to the other answer which redirects to sites like "Apps for Elementary" I want to keep an update on this, gradually adding here some info on the apps that I test.
There are these reviews on some of the apps on Youtube: Elementary 3rd Party Apps Reviews.
But that review, as well as the linked addresses seem to be from the Freya period.
In Loki some of the apps are part of the default ones, like Calcualator, or are very commonly used, like Tweaks.
Webby, the web app browser, has a ppa that doesn't work in Loki.
Translator uses Google Translate to translate between seven languages, works in Loki. Also includes a dictionary.
