Version control is an important part of any software project. We have always used a Software Configuration Management (SCM) system here at Softeq. This allows our developers to work on code from around the world.
At a minimum, this involves a product like Visual SourceSafe from Microsoft. Normally, VSS runs internally on a server, but web access can be provided by products like SourceOffsite by SourceGear. We have successfully used this combination of tools for many years.
Lately, we've managed several large projects using SourceForge OnDemand by VA Software. This is a private version of the open source system used at SourceForge.net. Many companies and teams can collaborate on the same private project and you have complete control over rights and access. Third parties can check in their own code yet only have visibility into their relevant portions of the project. Besides the code repository, OnDemand includes defect tracking, a task manager and a file release system. It also includes a Wiki system, but frankly the Wiki is the weakest part of the package.
If your developers are just working off of their laptop hard drives, without any common Software Configuration Management (SCM) software, do yourself a favor and check out one of these systems.