What Google Talk compatible iOS apps can I recommend to my friends?
Apple iOS
No official native solution by Google (except for the safari web-app with a subpar user experience)
Beware: All 3rd party providers seem to need your friend's plain google password to be able to offer push support (not just their app)! All have some sort of session timeout for push notifications (when the app is in background): Needs a relogin after that timeout (eww!).
3rd party apps I found:

IM+ Pro, free, ad supported, push with unknown session timeout
IM+, 9.99$, no ads and longer push session timeouts as above. Good reviews.
BeejiveIM for GTalk: free, push session timeout up to 7 days, no voice/video
Chat for Google Talk Pro: 0.99$, push with unknown session timeout, noticed bad reviews in Germany, good in US though

Windows Phone
todo
BlackBerry
has good support, download here
Symbian
todo
Bada
todo
Send me your findings, I'll happily include them here.