For synonyms and antonyms:
The Literatures and Languages Library circulates print dictionaries. To find a monolingual Spanish dictionary, search the Online Catalog using the subject heading Spanish Language Dictionaries (Tips For Searching the Online Catalog). To find a Spanish/English dictionary, search the Online Catalog using the subject heading Spanish Language Dictionaries English.
Most of these apps are free, but some of them have free versions with an optional choice to pay for more services.
Duolingo- A Free App! One of the best apps to learn languages, it uses a game-like way of teaching you lessons. You can learn many languages on this app, and even learn a language as a non-English speaker. This application is based on phone use, so you can take your lessons everywhere you go. One of the best things about this app is that many of the lessons are created by native speakers, so you know you can be assured you are learning usable and prevalent phrases.
HelloTalk- A Free App! This app makes practicing speaking many languages super easy. Users can look for native speakers and converse with them using a whatsapp-like chat that has both text and voice messages. Correction of the messages exchanged is easily done with a built-in tool that transforms the exchanges into tiny tutoring sessions. Also included with this app is an integrated translation system when those moments of uncertainty may hit you.
HiNative- A Free App! This app makes learning correct pronunciation a breeze. It allows you to get the benefits of getting in touch with native speakers without the hassle of searching for an exchange partner or having to schedule a chat. Anything is up for question here; you can ask for translations, input on pronunciation, or advice on cultural norms. You can also help fellow learners with the knowledge of your own language(s)!
MindSnacks- This app is only for iOS devices (Apple products) but it sure takes the gold in gamification of learning languages! MindSnacks teaches 7 languages and it does this by having eight or nine tiny lessons with games at the end of them that are designed to help you learn vocabulary, grammar, or practice your listening. The app monitors your progress so you can see how much more learning you need to obtain proficiency in the skills taught. The basic download is free but is limited, and the buy in option is cheap and opens up your options.
Babbel- This app is a paid "cousin" of Duolingo, though with 40 free lessons, it is a great option! Each class uses pictures to teach you vocabulary. Then the words you learned are used in related phrases and short dialogues adjusted to the the level you are at to build conversation skills. There are handy pop-ups that explain the most important grammatical points related to the learned material and if you are on a computer, they also include short cultural notes. You can also get separate packages devoted to improving specific skills such as grammar or vocabulary if you already have some experience with the language and only want to improve certain skills. Lastly, Babbel’s classes can be downloaded and kept offline for you to study.
Memrise- Memrise is the place to go for your vocabulary retention. There are a lot of courses on almost every language, including ones that are "fictional" (Klingon, anyone?). You have your options between more standardized courses based on popular textbooks, vocabulary frequency lists, and less expected vocabulary collections with naughtier words or translations of works. What makes this app great are memes and gamification. The app has a learning method that relies on creating funny or bizarre associations with the studied words. Memes come to play with this method of teaching, and the everyone in the community can add their own! Learning, revising and creating memes is a source of points that help you advance in the Memrise hierarchy of users (which ranges from Membryo to Overlord).
Great Ways to Learn Spanish Online
BBC Languages: Spanish--A comprehensive and lively collection of free audio and video courses for both beginners and intermediate learners.
Marco ELE -- An educational magazine that helps in learning Spanish as a second language. It has many great articles on the acquisition of language and analyzes resources and books that deal with learning languages.
About.com Spanish Language -- The website maintaned by About.com's Spanish Guide features a variety of language lessons, blog posts and several interactive elements including quizzes.
Spanish Blog: Language and Culture -- Great blog maintained by Transparent Language, a leader in language software. It has a staff of several writers and uses lessons, videos and even cultural anecdotes to help you learn grammar, vocabulary, pronunciation and more.
Destinos -- These videos teach speaking, listening and skills to understand Spanish through the telenovela or Spanish soap opera. There are dialects from Mexico, Spain, Argentina, and Puerto Rico that allows the listener to learn of the different Hispanic cultures. The videos are closed captioned in Spanish.
Coffeebreak Spanish--Excellent Spanish Podcast aimed primarily at beginning and early intermediate learners.
More Online Resources
FSI Language Courses -- Free Spanish audio courses taken from the courses designed by the U.S. Foreign Service Institute. It looks a bit stiff and formal but there are exercises, lessons and a wealth of audio files. There are many other languages available as well, not just Spanish.
Notes in Spanish and Notes From Spain --Sister sites that offer a wide variety of helpful resources for Spanish language and culture. NIS focuses on learning the Spanish language with a series of podcasts and NFS focuses cuisine, lifestyle and travel in Spain.
The Spanish Blog --A good resource put together by a Spanish teacher from Bilbao, Spain. Blog posts range from pronunciation guides to recipes and there are a wide variety of cultural and multimedia resources to help you learn and have fun at the same time.
Spanish Language and Culture--Maintained by a Spanish professor at Colby College, this website contains grammar exercises and some excellent activities which use songs, videos and even poems to help improve your Spanish.
Learn Spanish in Chile--Their Spanish Learning Area offers online quizes and games for practicing Spanish.
Study Spanish.com-- Features a wealth of tutorials for grammar, pronunciation, vocabulary and verbs available for free. Mostly for beginners and intermediate learners, with advanced lessons available if you sign up for their premium account.
Social Sites to Help Learn Spanish
Bitacoras and Meneame--If you've heard of the social bookmarking site Digg, these are two Spanish language equivalents. Search across thousands of blog posts and articles in Spanish, read the most popular posts on their homepage and vote for ones that you like. Great way to practice your reading skills and you might even meet some interesting new people. The two function almost exactly the same.
Tomisimo.org Forums and Wordreference.com Forums--Active language forums where you can post grammar questions to have them answered by native speakers, or you can learn by reading other threads or answering others yourself. You will almost always get replies by authoritative, knowledgable posters, which you can verify by seeing their native language, number of posts and reputation.