Eggs: You can't use this for a scrambled egg (unfortunately), but Ener-G Egg Replacer is perfect for baked goods.
Butter: Earth Balance has so many great varieties, but we have been using their Soy-free spread. Great for cooking, spreading on toast, baking, etc.
Cheese: I still haven't found an acceptable eat-in-large-chunks vegan cheese (though that's probably a good thing for my waistline), but Daiya is my favorite meltable vegan shredded cheese.
Sausages: I could live on Field Roast's Apple Sage sausages--even if I was still a carnivore.
Mayo: Another condiment where I can't tell the difference. I make a quick and easy sriracha mayo with this Vegenaise (1 T mayo, 1 t agave syrup, 1 t sriracha). But be careful--it's addicting!
Burgers: Because sometimes you just need a burger...and these beefless ones by Gardein really fill the void (of your mouth).
I can find all of these easily between Whole Foods and MOM's Organic Market (and most of them also in the organic/natural section of the larger, mainstream grocery stores like Giant and Safeway).
What do you think you would find the hardest replacement for you to go vegan?
I'm instagramming my food again (@mandamiska) for Meatless Monday, so come check it out (in a bit...breakfast is about to go on the stove as I finish this post!)