It is currently only illegal to melt cents and five cent coins (pennies and nickels for those of you not into numismatics). You can melt everything else. It's not cost effective to melt dimes, quarters, halves, or dollars though. The only exception is war nickels (minted from 1942-1945 with the large mintmark over the dome on the reverse). They're 35% silver and can be melted.

Plenty of people still melt silver dimes, quarters, halves, and dollars for the silver content.