Whilst mooching about I often take print-outs from the old maps website http://www.old-maps.co.uk/. Trouble is these tend to be rather flimsy or get wet. A while ago I discovered the Godfrey Edition maps, lovely old one inch OS maps for about three quid a pop. http://www.alangodfreymaps.co.uk/
My latest discovery is the Cassini Old Series Maps. They've taken the old one inch maps and enlarged and re-projected them to match OS 1:50,000 maps. http://www.cassinimaps.co.uk/shop/. At six pounds and forty nine new pence they are good value for money and could be a valuable addition to your fieldbag. The only drawback with them is that, unlike the Godfrey maps, they only cover England and Wales.
Has anyone else come across any similar resources?