What are some good API's to consider when starting development on a stand alone GUI applications?

For most of my career I've implemented client / server console applications, shell, server-side web applications and tons of Javascript. I know this is pretty generic but some of the things that are important to me:

- Cross platform compatibility
- Ease of end user installation
- Robust high level OO API
- Ability to prototype but not critical
- HTTP network interfaces /w cookies / sessions and ssl capable
- Good community support

I've looked at a few different GTK solutions like PHPGTK, pyGTK, Java and C#.

My core competencies are C, Python, PHP, Perl and Javascript on mostly Linux and Solaris platforms so it would be nice to not have to brain up on something completely foreign.

Could anyone recommend a good open source solution and briefly explain their advice?

Thanks!

if your goal demands cross-platform, java is a good answer, though GTK is good as well. Both have a pretty good cross platform history.