10l florists buckets - I'm hoping they'll help me produce some aubergines this year (fourth year lucky)...
Blue mushroom trays - line with paper and fill with compost to produce a seed tray
500ml/380ml plastic drinks bottles - great for topping canes to reduce the risk of poking your eyes out
Wood out of skips on business parks/industrial estates or from expanding Tesco stores or make friends with your local shop-fitting business - great for edging fixed beds, making giant compost bins or weighing down crop covers
Bricks/paving tiles from skips - useful for weighing down crop covers
Lengths of old water pipe - cut to 1.8m lengths, stick a bamboo cane up each end to anchor to the ground and create your own customised cloche structure for covering with fleece, netting etc.
Empty multi-purpose compost bags - great for transporting garden waste/ guinea pig hutch waste from home to allotment compost bins
Shop carrier bags for transporting harvested veg from allotment plot to kitchen
Our borough council every now and again off load redundant wheely bins which can be used as water butts (if they haven't had their bottoms cut off) or compost bins (if they have)
One I'm going to try later this year is to cut my leaky redundant water butt in half lengthways, fill the two halves with compost and create a couple of strawberry beds, finding some sort of structure to raise them off the ground with
And anything of organic origin can be recycled in the compost bin...