SpanishDict - Find out why SpanishDict is the web's most popular, free Spanish translation, dictionary, and conjugation site.
Google Translate - Google's free service instantly translates words, phrases, and web pages between English and over 100 other languages.
GoldenDict - The program has the following features: Use of WebKit for an accurate articles' representation, complete with all formatting, colors, images and links.
DeepL Translator - DeepL Translator is a machine translator that currently supports 42 language combinations.
Wiktionary - Open Source wiki-dictionary by the Wikimedia foundation
Anki - Anki is a program which makes remembering things easy. Because it's a lot more efficient than traditional study methods, you can either greatly decrease your time spent studying, or greatly increase the amount you learn.