I only have SSMS 2008 Express on my dev machine. What tools are there that I can use to develop, maintain, and manage server side SSRS reports on this machine?
You need BIDS - Business Intelligence Development Studio, which is the Visual Studio shell for developing reports that target SQL Reporting Services, and is included as part of Reporting Services install.
You can download SQL Express with Advanced Services from here. This version of SQL Express includes Reporting Services.