Wiktionary has an impressive collection of reconstructed Proto-Germanic words with a detail page for each root word (like e.g. this one) with a tree of the "reflexes" of the word in the Germanic languages, i.e. the descendants of the word in daughter languages.
The Cognate Checker below can produce lists of candidates for cognates (see the warning below) from all those Wiktionary word details pages.
You can produce lists of words of one language like English that have, according to said Wiktionary pages, cognates in a second language like German. Or you can produce lists of words that don't have a cognate listed in another language and may indeed have died out in that Germanic sister language.
The degree of coverage varies widely between the languages, not the least because "smaller" Germanic languages are attested and/or researched and documented much less than the main languages. English and German indeed work best here.
Caution: Why is it said above that this only produces "candidates" for cognates? Because there are (at least) four possible sources of errors where words will be listed (or not listed) as cognates without being such:
- The extraction of information from the Wiktionary pages is only maybe 90% reliable
- Words may not be cognates in the strict sense because there was loaning involved somewhere
- Some of the Wiktionary pages may contain errors
- The Wiktionary pages may ommit words