Depends on your budget. I worked in downtown, so I'll stick to those locations...
Joe Fortes does a Blue Plate Special at lunchtime for $10.
Vancouver Art Gallery Cafe is great - excellent balcony in the summer.
Brioche in Gastown is nice too for lunch.
Snow Garden is a sit-down Chinese restaurant (469 Richards), but they have cheap (less than $5) quality takeout, with a daily-changing menu. If you want cheap chinese - this is my top recommendation for downtown.
Flying Wedge Pizza is great value for fast-food. Try the Szechuan chicken pizza.
Kintaro in the West End is excellent Japanese ramen, but there is usually a line.