I have just released the Julie Resynth (31 instruments) and Square Wave (26 instruments) expansion sets for Thumbjam. They sell for $2 and $1.49 respectively.
Both have free demos downloadable from my website,6 free instruments for Julie Resynth and 2 for Square Wave. Visit this page for more info:
http://lesproductionszvon.com/for_iOS.htm
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I made 2 very simple preview videos of just browsing through some of the instruments:
http://www.youtube.com/watch?v=aj7MaTS8QUM
http://www.youtube.com/watch?v=5plVrUO1s64