Collins English and Thesaurus For US$39.99 you can have the ultimate language reference. over 500,000 English words and 377,000 synonyms with key expressions, examples sentences, culture, and usage notes. You can d/l the whole thing and use it offline. It's expensive, but it's all you'd ever need.

Oxford Dictionary Thesaurus I'm always a fan of the Oxford. This has 200,000 entreis and definitions and more than 300,000 synonyms. It's US$19.99 and you can d/l it for offline use. It's not as good as the Collins.

Advanced English & Thesaurus This is the best of the free thesaurus apps. Has definitions of over 140,000 words with more than 250,000 links and 1.4million words. D/lable.

Merriam-Webster Unabridged The best learning tool for English. It's US$59.95 and has over 257,000 entries, 700,000 definitions, etymologies and extended thesaurus. Also a bunch of learning tools including flash cards. Plenty of great search functions. Could work more consisitently though, not worth the money IMO.

Insult Thesaurus FREE At least I got the free version. The best part about this lame app is the logo.