My goal here is to point out places that you wouldn't notice, or expect, or something that's unusual. I'm partial to cheap, good places that are open really, really late.
El Vaquero 18th & Damen
As you might expect from its Pilsen location (18th & Damen), you get a taste of authentic Mexico. The menu is relatively standard, but this is one place that can get its spice on ... Close between 10pm-midnite daily. The quality of food is consistently very good, and the portions are sure to fill you. A TV plays on a spanish station in the background. If you come late at night, though, be prepared to orden en espanol. Prices are $4-7 per entree.
Cuernavaca 1100 block of 18th (between Racine & Halsted)
Even before I moved to Pilsen, this authentic mexican restaurant quickly became a favorite. Slightly mas dinero than El Vaquero (still everything less than $10), but this offers a bit more ambience -- and it actually feels like a sit-down restaraunt. There's a bar, too. One time when I went, they had live music. Excellent food.
Baba's Palace On Chicago, just east of Ontario.
This is the 1st 24-hour Indian restaraunt (hole-in-the-wall) I've seen. And it's also in the el cheapo price range. $4.50 fills you up. They have a daily rotating menu. The quality isn't that great. I went Friday at midnite, and it was packed. Looks like a cabbie hangout. If you go late enough, parking isn't a problem (meters stop at 6 pm). Thanks to Best of Chicago for this one...
Devon Ave between California & Oakley.
The India-town of Chicago is best visited for lunchtime buffets. Favorite North Indian - Tiffin's. There are 3 or 4 South Indian restaraunts, too. There's even a Gujarati eatery (Annapurna's)! Poke around for yourself.
Little Italy Taylor St, from Halsted to Western. Nestled in between the two UIC campuses, Italy got squeezed out by the rest of the world. My personal favorite is Rosebud, though some are partial to Francesca's (mucho $$ and really really nice, but actually OK once you consider one plate easily feeds 2 starving students). Thai - Golden Thai. Bars - sick of 'em all (you'll find M1s at Hawkeyes, M2's at Bar Louie, and grad students at Illinois Bar. Middle eastern food is good at Couscous though the quality is variable and often reflects a new ownership.
If you cross Damen, be sure to check out Bacci's pizza for their $3 slices. One slice easily feeds 1 starving person or 2 normal people. The pizza ain't great, but hey ... Some of their other entrees are actually pretty good.