Think "Google" as a verb. "iGoogle" means, "I, Google".[1] (Same logic can be applied to the iPhone)

According to iMac's history on Wikipedia[2], "i" stood for "Internet". (Again, same logic can be applied to the iPhone and iPad. No idea about iPod, which came after iMac but before iPhone and iPad)

[1] This is my personal opinion. [2] http://en.wikipedia.org/wiki/IMac#History