Hmm,maybe this is not easy like you think, why because any designer web have one and more tools for it :-). For me,

• Adobe PhotoShop for make web mockup.
• Color Schamer for make color combinations
• TopStyle Pro for Css code Builder
• Some Browser like IE , FF, Safari, Mozila, Opera, Flock for CSS layout tester.
• DreamWeaver for Code like html, php, and more.
• AppServ For my lover local server.
• Aqua for drink
• Sedateness for my brain.