Can anyone tell me the difference between these repositories
lp:openobject-addons
lp:openobject-addons/extra-trunk
lp:~openerp-community/openobject-addons/trunk-addons-community
openobject-addons contains the "certified" addons, maintained by OpenERP SA.
extra-trunk contains 3rd party addons. It was split from openobject-addons in 2008. The quality of the addons in there varies greatly and some of the addons are not maintained at all, but a lot of them are useful. To contribute to that branch, you need to do a merge proposal and get it approved by OpenERP SA.
trunk-addons-community seems to be a recent branch containing new addons. You can contribute to it if you are a member of the openerp-community team.
The current trend is to move the most useful addons from extra-trunk to new branches with a smaller focus to ease maintainance. You can browse the latest addons at apps.openerp.com, and you can also register your own addons.