What tools do you use to develop Oracle stored procedures, in a team :
Treat PL/SQL as usual code : store it in files, and manage these files with your revision control tool and your internal procedures.
If you do not already have a revision control tool, then write your requirements down and pick one up. A lot of people it seems use Subversion, associated to TortoiseSVN as a client on Windows (I do).
The thing is : use your tool as is recommended, and adapt your procedures accordingly. For instance, Subversion uses a copy-modify-merge model by default, as opposed to a lock-modify-unlock model which you seem to favor.
In my case, I like to use TortoiseSVN, as stated above. And as is usual with this tool :
And whatever your choice, I recommend reading this post (and related ones) about database versioning.