DICT.CC – my favorite, although I use both this one and Leo (below) when I'm translating. I used Leo for years and also love it!

LEO – Seemingly everyone's favorite.

BEOLINGUS – A student with a technical background prefers this one. Maybe you do, too?

COLLINS – German and four other languages.

Abbreviations – very important in German! – From Abk. (Abkurzung) to z.B. (zum Beispiel).