The key to any successful development team, far more than any other tool, is version control. No matter how many team members you have, if you're dozens of people spread across the globe or one lone developer in a basement you need version control.
A number of free and paid for systems exist, largely they accomplish the same ends: the ability for multiple developers to collaborate on the same code, the ability to track changes and the ability to manage changes across concurrent versions of code.
This is potentially the biggest benefit of any source control system, the ability to manage changes in code. Most version control systems manage changes through a series of checkouts and commits by developers.
You check the code you want to work on our of the repository creating a local copy, make changes to the code and commit your changes to the repository again. Any developers that are working on the code can update their local copies an the changes you've made will be merged in.
This can be a massive boon, even if you're working alone. Version controls systems will also let you roll changes back, so if you've made a mistake, deleted a file or need to see how something worked before you can go back to any previous version.
Concurrent versioning is a great benefit when you've released software or are developing more than one major feature at the same time. It's the process of having two working versions of the code which can be edited independently.
In the case of a released version of software this allows you to release a patch before new features still in development are ready to be released. You can patch the bug, release an update to users and then use the version control system to merge the change into the development code so that your next release with the new features gets the fix too.
If you're working on a major feature you might also want to branch your code temporarily. This would allow you to perform some pretty significant refactoring on areas of the code which would not affect developers working on the rest of the code, when you finished the work you could then merge your code into the main development branch prior to release.
How to get started
There are a number of choices and a number of web based version control systems. Opensource subversion and git are used by a large number of people and compete on features with most commercial offerings. If you don't want to set up your own server to host a repository Google code, beanstalk and lighthouse are just a few web based services that can host version control environments for you. Google code is free, but you'll need to be developing an opensource project the other two are subscription services.
I personally use subversion and love it but with any version control system, commercial or free the most important thing is to pick and fit a system into your development environment. Don't pick one that is going to force you to change the way you work. Try a couple if you can and pick the one that works best for you.