This is a national chain (started in Colorado) but it has only a handful of locations nationwide, with several being here in Chicago.
All-vegan menu. Ordering is at counter before sitting down. They have a great beverage selection that includes a few bottled beers as well as house-made lavender lemonade and hot chai.
While the Chicago Diner proudly proclaims it's been "meat free since '83," Original Soul Vegetarian has been providing vegan food since 1980. Everything from their seitan ribs to perfectly tender (but never mushy) collard greens, is delicious. Hours and info on their website: