There were four voiceless frikative consonants in Proto-Germanic – /f/, /þ/, /h/ and /s/. These voiceless frikatives changed into their voiced counterparts – /v/, /ð/, /g/ and /z/ – with one exception (which was discovered by the linguist Karl Werner): They remained voiceless when they were immediately preceded by the accent. This happened before the accent moved to the first syllable in all words.
An example: Long before the accent was fixed to the first syllable in all words, the PGerm. verb *wesana 'to be' and its singular preterite forms, like *was 'he was', had the accent on the first syllable, whereas the plural preterite forms, like *wêsun 'they were', had the accent on the second syllable. In *was the /s/ was not changed, because it immediately followed the accent, but in *wêsun the accent was elsewhere and therefore the /s/ turned into a /z/.
This gave rise to an alternation that is clearly visible in the inflection strong verbs in Germanic languages:

	infinitive	pret. sing.	pret. plur.	perf. ptcp.
Old Saxon	heffian 'lift'	hóf	hóbun	gi-haban
Old English	líðan 'go'	láð	lidon	liden
Old High German	slahan 'beat'	sluoh	sluogun	gi-slagan
Modern English	(be)	was	were	(been)

In ON. the alternation has been evened out in many of the affected verbs. The alternations /f/ : /v/ is nowhere to be seen, but the others can still be seen in a few words.
The alternation /þ/ : /ð/ is seen only where /þ/ was assimilated with the preceding consonant, and /ð/ was not. The PGerm. *finþana 'to find' - *fanþ - *funðun - *funðenaz has become the ON finna - fann - fundu - fundinn.
The alternation between /h/ and /g/ has in ON become an alternation between nothing and /g/, since most /h/:s have disappeared. The PGerm. *slahana 'to beat' - *slôh - *slôgun - *slagenaz has in ON become slá - sló - slógu - slaginn.
Others: flá 'to skin', sjá 'to see', þvá 'to wash'.
The /s/ : /z/ alternation has turned into an alternation /s/ : /r/, and *wesana 'to be' - *was - *wêzun - *wazenaz has in ON become vesa - vas - váru - verit (neut.).
Others: kjósa 'to choose'.