This is a good one! I won't even go into all the crazy places I've had to prepare for a show, but I personally go through the following if I only have a small space:

warm up my feet and ankles (circles and roll throughs)
calf stretch and quads (and I have figured all kinds of different ways to do this depending on the amount of room) =P
figure eights to stretch out my hips and warm up my obliques
gluts (with level changes if space allows), twists, pelvic locks, a few squares & circles
large hipcircles
ribcage isolations (side to side, locks, circles)
Full body sways
Forward bend and back bends where I can get the space
Arms, wrists, and hands (snake arms side if possible, down or up if needed, wrist circles, and hand undulations)
Shimmie

Just a few of each depending on the amount of time available. If I'm on standby, I just keep repeating to stay warm and limber.

Ha! My troupe's most recent performance found us warming up/practicing in a narrow hallway which was the exit to the trash bin and located next to the men's restroom! It was the ONLY space available away from the main performance area. Talk about inconvenient. Then the photographer for the event discovered us and BEGAN TAKING PICTURES! Ugh, not the best light in which to present your troupe, to say the least....

Regardless of space, I always warm up all my joints with slow circles and gentle stretches, especially my lower back, hips and shoulders. While I'm doing this, I "warm up" my mind as well, doing a mental practice of my performance including how I will breathe during each move. Sometimes, we forget the importance of warming up our brains, as well as our bodies =)

When confined, I try to make use of the walls in front of me, door jams, etc. to assist in warming up.

stretch a foot up the wall to help open the hips, then switch
push off away from the wall to warm up arms
one hand on each side of the door jam and lean forward and backward to open shoulders and chest and back
"cobra" stretch up the wall (so the rest of you is against the wall, shoulders and head leaning back
side stretches, making use of the wall – face the wall, walk hands sideways, making sure torso stays against the wall
lunges with front leg against the wall, stepping back from it
forward bends with you butt on the wall

and more. Use what you have. When I have plenty of room, I do my Tai Chi to warm up, including the meditation if there's time. Sends energy to every muscle, every joint and wakes up the mind.

I often do the salutation that i begin/end classes with--it's slow, meditative and gets my body in the flowing/beautiful mode.
If there's space, I like to do a few crunches or sit ups to get in touch with my core, so it's warmed up a bit before i go out.
I swing my arms to get the blood pumping.
Forward folds do wonders for me (before a show, or anytime in 'real life').
If i'm dancing to canned music I like to listen to my ipod and mark it several times before i go on.
I consider anything else bonus--but I like bonus!

If I'm contorting, I have a much stricter requirements--at least an hour of deep stretching.