Since WordNet is open source, there's lots of on-line dictionaries and thesauruses that use the WordNet database, but they often lack the vital links between word senses that makes the dataset useful, preferring to implement cool visual effects instead. I've just found an on-line interface to WordNet that has the vital semantic links: http://www.wordnet-online.com .
For example, take this link for the word "anxiety": http://www.wordnet-online.com/anxiety.shtml . You can follow links downward to more specific terms like "panic" or "hypochondria". Or you can follow one of the "is a kind of" links up to "mental state", where you'll find terms like "anhedonia", "hypnosis", or "enchantment".
That's an example of "ontology", or "kind-of" relationships. But WordNet also captures mereology, or "part-of" relationships, and "instance-of" relationships.
Check out the entry for "battalion": http://www.wordnet-online.com/battalion.shtml . From that you can glean that a battalion is not only a kind of "military unit", it is part of a "regiment" and has one or more "companies" as parts.
The mereology links aren't perfect; you have to look up "division" (the unit above regiment) manually, and from there you end up in an alternate part-of hierarchy (corps > division > battle group > company > platoon). Likewise "squad" is not connected to "platoon", and "fireteam" is entirely missing.
Although the database is imperfect, I find the linkages useful when I need just the right word for "fear" or "anxiety".
Didn't realize there were apps for this sort of thing, but I'm so technophobic I've only just progressed to a Nook Color---my cell phone, kept in the glove compartment for emergencies, and taken on vacations for the better long distance rate, is old and designed just for making phone calls. I suppose I should upgrade, one of these days, real soon.
[This message has been edited by extrinsic (edited September 20, 2011).]
I should note I don't think these things are a replacement for a synonym dictionary or thesaurus, but the navigability of the interface makes them a handy aid for sharpening up a fuzzy picture. A character is fearful -- no, make that anxious... no, apprehensive. A character has some disease... rheumatism ... he is arthritic. The woman is blond, towheaded, no, make that ash-blond. A skip through the complexion link takes us to tawny. I take that back; her complexion is the creamy white of a willow lathe.
[This message has been edited by MattLeo (edited September 21, 2011).]