Caustic is cool, but there's a bit of a learning curve. I like my android apps super simple and put through external effects, and I don't need anything nice, musical, or professional-sounding. All I really want out of my android is for it to be a good noisemachine, so I use
Grain Synth,
DroidBeat and
Flyby Synth - all ridiculously loud and obnoxious free android noise apps that make great base sounds for mangling though pedals and racks and stuff.
Fart Synthesizer is a staple, especially if you have any young kids in your life, they will absolutely love it and laugh for hours.
Favorite free iphone/ipod touch app is Alchemy, but like most things, it's better on the ipad. Other free shit currently on my ipod touch: Arpeggionome, SoundPrism, Thicket, Anaphobia Mini, Pollisynth, Bucephalus, Ultimate Beat Maker Dubstep edition, Waviary, Beatwave, Mixis, Fourier Air & Touch, Junglinge, DnB Machine, Euphonics, MegaCurtis, and a whole bunch of free soundboard/sound button apps (animal sounds, robot sounds, scifi sound effects, horror, halloween etc)
For the ipad, Waldorf Nave is keeping me pretty enthralled. AniMoog, Thor, and the Korg and Arturia sets are all super tempting, but Nave is deep enough to be my main ipad interest for now.
TF7 is a good free FM synth for ipad, but Alchemy is the best free synth by far IMO. Easy to use and tweak, fun, versatile, and it has transformable beats.