Development question regarding instant massenger

[avipronyc]

Hello,

I am looking to develop a software that could be integrated into AOL, MSN, Skype, Gmail and Yahoo messangers.

I was wondering:

1. on what platform those IM's developed?
2. on what platform should I develop my own software, that would be easy to integrate to
those IM's?

Note: my software should be executable when installing and integrating it as part of those IM's.

Thanks for your help.