Check out the sites for AIM, Yahoo and MSN. Most have a short number service you can text. For example, here in the US you can text 92466 (YAHOO) to sign in, sign out, and IM friends.
I don't know of any apps that do it right now (maybe Pidgin has an extension for it?), but a quick QT app could probably use the interface pretty seamlessly. Yahoo is nice in that it assigns a long number with a custom LD code (15 digit number) to each friend in your list. So you can save that as a unique texting number per friend.
just want to know, so I have n900 with t-mobile talk+text; and I want to use the IM (msn, yahoo and AIM)
i used to IMing with my old phone with the apps that uses the text messaging service.
but since moving to the n900, I can't get online with only the text messaging anymore.
which means i need the data plan. but i dont need the data plan aside from IM ing.
does anyone know what program to use so that i can IM ing with my unlimited text plan?
thanks