There are hundreds, if not thousands, of really useful websites and any website will lead you to many others. Only the most significant in terms of plastics in the museum context are given here. Although these are grouped under a particular subject, they are often also a source of information about the other subjects, too.
General
http://www.bpf.co.uk/Plastipedia/Default.aspx
Site of leading trade association of the UK's plastic industry. Wide-ranging but especially good for materials and processes, the latter accompanied by animations of the processes in action.
www.plasticsresource.com
American Plastics Council site, with succinct history of particular plastics up to 1950.
www.plastiquarian.com
Excellent information packed site of the Plastics Historical Society, especially good on the history and uses of plastics and their inventors/manufacturers (up to 1965). Includes useful index of tradenames/materials/manufacturers. If you are only going to look at one site, this is likely to be the most useful.
Care
https://www.nfsa.gov.au/preservation/preservation-glossary/storage-cellulose-acetate
Good for information on and products relating to absorbents and buffers.
http://nfsa.gov.au/preservation/handbook/
Helpful guidance on the care and preservation of film based polymers.
http://www.conservationregister.com/PIcon-careplastics.asp
Succinct account of plastic conservation issues with access to practicing conservators.
Objects
www.deutsches-kunststoff-museum.de/
Site of the German Plastics Museum. Objects grouped by materials and themes with good images and texts. A little hard to navigate but worth the effort.
www.kartell.com/gb
Site of the Kartell Museum which now includes more than 1000 Kartell products.
http://museo.cannon.com/
Site of the first Italian Museum of Plastics, founded 1985. 2500 well-catalogued objects presented informatively through a range of themes.
http://plastics.syr.edu/
Site of the Plastics collection at Syracuse University Library, New York, USA. Currently developing an on-line collections database: excellent so far as it goes but currently limited, plus useful timeline.