Cook pasta in plenty of boiling salted water until al dente, 7-10 minutes; drain.
Meanwhile, melt butter in a large skillet over low heat, stirring until creamy.
Stir in lemon zest and juice.
Toss lemon sauce with hot pasta; serve.
*Spaghetti with Lemon Butter and Tuna: add 1 (6 oz.) can chunk light tuna, well drained, and 2 tablespoons coarsely chopped flat leaf parsley to the lemon sauce; heat gently 1 minute before tossing with hot pasta.
*Spaghetti with Lemon Butter and Peas: add 1 cup frozen peas to the butter; heat, stirring, over low heat until peas are tender, about 3 minutes; add the zest and lemon juice; toss with hot pasta.
*Spaghetti with Lemon Butter and Shredded Zucchini: shred 2 small zucchini (about 6 ounces); add to butter; heat, stirring, until crisp-tender, about 3 minutes; add the lemon zest and juice; toss with the hot pasta.