I'm super enthusiastic about this wonderful business suite and i want to collaborate with you guys! How can i do?
check out this link: http://doc.openerp.com/v6.1/contribute/index.html#how-to-contribute-link and you'll find plenty of information on how you can contribute making OpenERP a better product!
Since the answer from Quentin is quite old I decided to add the 'new' wiki. This explains how to report issues, how to handle tracebacks, how to make PR's (and against which version) and info about translations/labels.
You can find all the information at https://github.com/odoo/odoo/wiki/Contributing
Please note that to be able to have all the copyright of the openerp server and bundled modules. Openerp SA do not include external contributions to the source. (Correct me if I am wrong.)
Personally I do contribute by translating OpenERP into my language and localization.