Agile Resource Planning at a Fixed-Bid Drupal Dev Shop

On any given month, Metal Toad Media will distribute the work for 25-30 different clients across 10 different development resources. The majority of our work comes from fixed-bid contracts, but we also have a number of time & materials maintenance contracts to fill in the gaps. We've tried many different software solutions over the years to coordinate this work, but we've settled on using our own set of customized Google spreadsheets to do our long-term resource planning and short-term work allocation in a very dynamic and agile fashion.