SpanishDict, Word Reference, and Google Translate are all useful online tools for translating words into Spanish. SpanishDict translates any English word to Spanish, while Word Reference can translate French and Portuguese as well. Google Translate can translate from many world languages into Spanish.
SpanishDict offers a simple yet thorough definition for each word, with compound forms shown under the definition. A tab shows multiple examples of the word used in a variety of contexts.
Word Reference offers its own free Spanish-English dictionary, with compound forms and idioms and expressions also translated. A separate tab also shows the Collins Dictionary definition.
Google Translate is the most instantaneous service, as the translation of what is being typed appears on the same screen, with definitions at the bottom. All three dictionary/translation services make use of both traditional dictionaries as well as user-generated examples and translations.