The University of Chicago library's research guide for Germanic Studies offers many helpful links in one place, and it provides special access for UChicago students to paid resources like the MLA International Bibliography and the BDSL (see below).
The Bibliographie der deutschen Sprach- und Literaturwissenschaft (BDSL) is the most important bibliographical source in Germanic Studies. The BDSL comprises world-wide coverage of published literature on the German language, literature and general Germanic studies. Open access search is available but limited.
The MLA International Bibliography provides a classified listing and subject index for books and articles published on modern languages, literatures, folklore, and linguistics for paid subscribers (for UChicago students with a C-net ID).
The University of Chicago library's research guide for Judaica and Hebraica (many resources are accessible only as paid subscribers, for example with a UChicago C-net ID).
Karlsruher Virtueller Katalog is a "meta-catalog" that allows simultaneous searching of multiple library catalogs.
Germanistik im Netz is a vitual research library for German language and literature. (German)
The German National Library in Leipzig is central archival library and national bibliographic center for the Federal Republic of Germany.
The Deutsche Internet Bibliothek is a cooperation of over ninety German public and research libraries and provides a vast link database, divided into twenty categories such as "law," "family," "history," or "literature." (German)
The "hzb Werkzeugkasten" of the Hochschulbibliothekszentrum (HBZ) of North Rhine Westphalia provides a comprehensive annotated database of links relevant for research, such as article databases, bibliographies etc.
The Zeitschriftendatenbank provides the opportunity to search the periodical held in German libraries. (German)
Bibliotheken, Bücher und Berichte offers a comprehensive collection of academic library catalogs in the German-speaking countries. (German)
The Deutsches Literaturarchiv Marbach is one of the most significant literary archives worldwide. (German)
The Archivschule Marburg offers a list of archives in Germany, Europe and overseas. (German)
The Digitale Bibliothek Deutscher Klassiker provides access to a premier collection of German literature (restricted access)
Monolingual Language Resources
Digitales Wörterbuch (DWDS) Digitales Wörterbuch der deutschen Sprache des 20. Jahrhunderts (German)
Jacob and Wilhelm Grimm's Deutsches Wörterbuch (German)
Dictionary.com English-language Dictionary, Thesaurus, Encyclopedia
Online Etymology Dictionary English etymology
Urban Dictionary Slang dictionary based on user contributions
Bi- or Multilingual Language Resources
canoo.net offeres a large variety of free online German language resources, including monolingual and bilingual dictionaries in several languages, word and sentence grammar, word formation rules, morphology, a linguistic glossary, spelling rules and spell checker.
woerterbuch.info Online Dictionary German-English, ca. 750,000 entries
dict.cc Online Dictionary German-English based on user contributions, ca. 500,000 entries (offers also synonyms)
LEO Online Dictionary German-English, ca. 430,000 entries (offers also German-French and German-Spanish)
Beolingus Online Dictionary German-English, ca 325,000 entries (offers also German-Spanish)
odge Online Dictionary German-English, ca. 210,000 entries
Ultralingua Online Dictionary, multiple languages, ca. 350,000 entries each
Yiddish Dictionary Online, Yiddish-English