kgs is the obvious choice for native English-language
speakers (and others -- several spoken-languages are
represented there). NNGS has a critical shortage of
people using it (often less than a dozen present at
any one time). IGS and KGS are generally well-populated,
though certainly not at every ability-level. A double-
digit kyu player (self included) will find any of
them challenging (a game you may win or lose, rather
than one you pretty much know you will win; or lose!)

I don't think the Palm device is well-suited for Go
because of the form-factor; it is difficult enough with
a sizeable display. I have one or two game recording
(SGF-generating) Palm applications, but I've yet to
use them even once under the pressure of playing a
real game (I wouldn't even want to record as an observer
using only a Palm device and any software that might
run on one). Perhaps a much younger person's eyes
might find the form-factor useful -- I find it to be
less than useful here.

I've heard there are several non-English Go servers
(notably in Japanese languages, or Korean languages)
that are great for native speakers. IGS and KGS
seem the only really useful choices for real-time
play on the Internet.

The yahoo go server is the best for it is quick.
My fav book is The Direction of Play