Groundwork
I've not attempted too many bases for my finished models, and the ones I have had made have been minimal affairs utilising items I've found about the house. Old wooden name plates and even an Ikea shelf! Not the best looking or ideal material but they have been a test bed and cost me nothing.
I have been slowly acquiring various groundwork products such as cheap picture frames, clays and sculpting materials, vegetation, ground mats and the like. To date I have only used a few of these materials, and the bases built from them have been very small affairs, showing simple road or track sections. As for buildings I have messed around with a selection of Miniart's vacuum products but didn't get on with them too well!