G3 String 196Hz

D4 String 293.7Hz

A4 String 440Hz

E5 String 659.3Hz

Click on the note to hear the pitch.
Source: Violin Online - Violin Tuning

Tuning Apps:
Tuner - gStrings Android free
Instrument Tuners For The iPhone: iPad/iPhone Apps AppGuide

Links:
3 Ways to Tune a Violin - wikiHow
Violin Online - Violin Tuning
Note Frequencies
How to Tune the Violin - Get-Tuned.com