Where can I start to learn about PLCs?

Try the following web sites for basic training (some have PLC simulators available):

http://www.plcs.net
http://www.mrplc.com
http://www.thelearningpit.com
http://www.plcdev.com
http://www.industrialtext.com/freestuff.htm
http://engineeronadisk.com/V2/book_PLC/engineeronadisk.html
http://www.plcman.co.uk
http://www.koldwater.com/
https://www.plcopen.org/
http://www.youtube.com/RonBeaufort
http://www.plcmentor.com
http://www.plcprofessor.com/
https://www.theautomationstore.com/lessons/

Site links may change without notice, so general sites are included as opposed to specific product links. Most require you drill down to find the product information you desire. Some of the following links refer to a company's U.S. site.

For manufacturer-specific information:

Allen-Bradley: http://www.ab.com
GE: http://www.geautomation.com/products/programmable-automation-controllers
Automation Direct: http://www.automationdirect.com/adc/Shopping/Catalog/Programmable_Controllers
Mitsubishi: https://us.mitsubishielectric.com/fa/en/products/controllers
Omron: http://www.ia.omron.com/products/category/automation-systems/index.html
Siemens: https://new.siemens.com/global/en/products/automation/systems/industrial/plc.html
Modicon: http://www.schneider-electric.com/products/ww/en/3900-pac-plc-other-controllers/

If anyone knows of sites I can add to this or if links disappear, please let me know. Thanks.