Roundup is pretty good for spot-treating individual plants you don't want (dandelions on paths and in lawns and the like). Simazine (various brand names) is good for treating large areas like gravel drives, but often doesn't kill off the root of big, tap-rooted weeds like dandelions once they've got established. "Lawn sand", containing ferrous sulphate is useful for discouraging broad-leaved weeds and moss in lawns.
Depends what you want it to do exactly.
In fact, last time I looked, there weren't many other active ingredients Joe Public could buy for amateur garden use.
Boiling water or a blow torch are pretty effective if you don't want to use chemicals and there aren't any other plants nearby you don'twant to kill.
Whatever you use, this time of year (ideally a bit earlier, before most weeds got a first crop of seeds off) is a good time to use it, when weeds are growing at full speed.
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