Is a silver certificate still legal tender?

Technically ALL American bills ever issued except for gold certificates remain legal tender. The government has never withdrawn any of the designs.

However most later-date $1 silver certificates are in fact so common among collectors that they aren't worth much more than face value anyway.

Other older bills may or may not be worth more than their face value; you have to know their dates, series letter (if any), and seal color for starters.