Aside from the obvious, like Wiktionary, Duden, there is also:

• Verbforen – generates German verb forms using smart rules. Proprietary.
• OpenThesaurus – mostly synonyms, German.
• bab.la – verb tables in different languages, proprietary.
• Morphologie-Lexikon – generated using Morphy.
• Wortschatz Uni Leipzig – SOAP API, pretty scientific dataset.
• Wordnik – closed db, supposedly from Wiktionary, API, English only, interesting API Showcase.
• LanguageTool – Open Source style and grammar checker, with a builtin database.

Attempts to parse Wiktionary (collected from SO)

• Dbnary (open, multiple languages)
• wiktionary-parser (open code, unfinished)
• jwktl (open code, seem quite extensive)
• wikokit (open source, Russian and English)
• perl-wiktionary-parser (English only)
• dbpedia RDF extraction (open code, seemingly API in the future, declarative extensions)

Part of Speech tagging

• wikily-supervised-pos-tagger