Online Shruthi:
http://www.karnatik.com/shrutibox.shtml
http://www.anubodh.com/tanpura_for_bansuri.htm
Shruthi Apps: itanpuralite & realtanpura
ITANPURA LITE
http://upasani.org/home/iTanpura.html
Download at your app store!
One more App:
Shruti box tambura –
http://itunes.apple.com/us/app/shruti-box-tambura/id490452104?mt=8