Most folks seem to be heading to the Maine coast, but for those driving up interstate-95, I have a few recommendations in and around Waterville to whet your appetites.
Appolo's Bistro (Silver Street, Waterville): Continental cuisine. Well prepared and presented food in an elegant setting, uneven service. The lamb is good enough to every time. The short menu changes weekly or so.
Breadbox Cafe (Downtown Waterville): Started by a former cook at The Last Unicorn (see below), the Breadbox is an upscale but informal spot offering a range of excellent dishes. Good wine list, memorable desserts. Waits are common.
The Last Unicorn (Downtown Waterville): Hands-down the best menu in town, offerings range from the ordinary (likely the best burgers in town) to the unexpected (crab cakes in Thai chile sauce). Typically, there are 20+ specials to complement the already extensive menu. Service is friendly if over-busy, atmosphere is very informal. Waits are expected.
Freedom Cafe (Downtown Watervlle): Honest-to-goodness southern cooking in Maine. Fixed price dinners include an entree, vegetables, bread, iced tea or lemonade, and your choice of cake or pecan pie. The chicken fried chicken and ribs are excellent. Not fancy, but filling. Thurs-Sat, waits are expected unless you arrive early.
Roy's Barbecue (Kennedy Memorial Drive, Waterville): Serving extraordinary barbecue from the parking lot of Aubuchon hardware during the summer months. Hope you like ribs, pork, and chicken with lots of sauce because that's all there is, but you'll be back for more. Cheap, but you eat out of styrofoam.
Grand Central Cafe (Main Street, Waterville): Unusually good brick-oven pizza with plenty of creative combinations compensates for bad service and a yard-sale interior. Skip the buffet.
Asian Cafe (Bay Street, Winslow): Unexpectedly good Thai cuisine -- the best in the area (Thai Star in Augusta is also very good). Excellent curries, rather nutty pad thai, very nice presentation, friendly waitstaff. Reasonably good Korean offerings (be bim bop), but pass on the Chinese and Japanese.
Pad Thai (College Ave, Waterville): Smaller potions and far less creative than Asian Cafe, Pad Thai nevertheless serves the expected fare at cheap prices. A favorite with college students.
Eastern Dragon (Temple Street, Waterville): Authentic Fujian (think Cantonese) and Vietnamese food (the Chinese owners spent much of their lives in Viet Nam). Extremely friendly owners may become overwhelmed during a rush.
Riverside Farm Market (Fairfield Street, Oakland): Memorable salads, sandwiches, quiches, soups, and baked goods served al fresco. The market also offers a range of gourmet foods to go, including a good selection of wines. Lunch only, closed during the winter.
Other people have recommended Port Rouge (Waterville - continental, leans toward vegetarian), the Village Inn (Belgard Lakes - duck, duck, and duck), and Bolley's Famous Franks (Waterville - hotdogs). I am also curious about the Sebbatis Smokehouse (Fairfield) if anyone has a review.