One of the most basic steps in proper weed control, and certainly in an Integrated Pest Management program, is to properly identify the pest you are experiencing. Here is a list of web resources that will help you identify your weeds.
Weed list for the UK, turf specific
Weed list for much of the USA(turf specific) and some non-turf specific here.
Weed list for Australia (NSW, non-turf specific)
Weed list for Ontario
Weed list for British Columbia