Here's my picks for the most authentic and popular restaurants of Korean friends. I left Koreana out. It's good but too "commercialized" these days.
Buk Kyung II - Korean Chinese Food
Colors - Casual Korean (cheap eats!)
New Jang Su BBQ - In Burlington, less crowded than Koreana
Min Sok - Specialize in sushi
Wu Chon House - Somerville
NOTE: 6/29/20008 Colors was damaged in a fire and is currently closed! We drove past and were shocked, though the damage didn't look too bad. I hope it reopens!
Be sure to leave a comment and let me know if I've left anything out! :) I try to list restaurants with authentic food that comes close to what you would find if you actually visited the country!