Find a project that you already use or that you like that supports plugins.
Write one plugin and learn by looking at other plugins.
On the top of my head:
deluge (bittorrent client)
emesene (MSN client)
XBMC (media center)
If you're a Gnome user, most applications support Python plugin (Nautilus, GEdit, ...)
Shameless plug on my part, you could make a plugin for my open source project called periscope. It downloads video subtitles from various websites
http://code.google.com/p/periscope/
On the top of my head: deluge (bittorrent client) emesene (MSN client) XBMC (media center)
If you're a Gnome user, most applications support Python plugin (Nautilus, GEdit, ...)
Shameless plug on my part, you could make a plugin for my open source project called periscope. It downloads video subtitles from various websites http://code.google.com/p/periscope/