Grackle

The easiest way to tell if you're seeing a Grackle is to wait for the cackle. Odds are also good if there are no other similar birds within viewing distance you may want to take a closer look because they're such social birds it's rare to see one without another.

Grackles are the big, dark birds that can flock up into the thousands and are really annoying when they all make noise at once, and if they're flocking in great numbers they usually are.

There are two kinds of Grackle here in Texas (three if you count the shoreline variety, but since I'm not near there I'm not going to) the Common and Great-tailed.
The Common Grackle is the dark grey bird with bright shiny blue/purple head and blue/purple edged wings and tail. This is the smaller of the two species, being only 11-13 inches long.
The Great-tailed Grackle is what I've been seeing a lot of. There's a disparity in looks between males and females in this variety. The males are black with a purplish gloss and up to 17 inches long, the females are brown with a paler breast and about 13 inches long. They also have thinner beaks and longer tails than the Common Grackle.

But general Grackle identification calls for knowing something that both these species have in common.
They: appear almost stretched, like someone took a regular plump bird and elongated it; have long thin tails (1/3 - 1/2 of their entire body length) that can fan out about 90*; have beaks that are long and pointed, which is good for their omnivorous life (they'll eat anything, even other birds); have shockingly yellow eyes as adults. As Grackles are social birds they will frequently nest with or near other Grackles.

But like I said, their harsh call is the easiest way to identify them. It can be like rusty swings or a rusty frog. Here's a nice example (scroll down a little, it'll be under "keys to identification" on the left side, it's a fairly small bar with a play button next to it). I'm hoping I can get a video of them so I can put it up here instead of linking to someone else's grackle noises.