Subversion is a version control system allowing developers to checkout projects, then to update the repository with there changes. Version control is an essential tool when working in groups (personally I use it even when working alone).
There are many issue trackers around (I use one called Jira by
http://atlassian.com, though there are plenty of free ones). These allow you to post bugs, feature requests, proposals whatever.... and have each feature (whatever) tracked through various stages of implementation.
Again, these are another way of making sure you don't tread on each others toes. A dev can login, assign a task to himself then go about there work knowing that know one else is doing the same task.