anything, use your imagination.
I piled textbooks ontop of a desk when I take class pictures for our yearbook.

I carry around and use the Gorillapod:

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I love it, it’s very useful, it allows me to get shots that I couldn’t get otherwise.

I’ve used all of these:
Railing
Tabletop
Beanbag
Rolledup jacket or sweatshirt
Book(s)
Backpack
Camerabag
Car (hood or roof)
Chair
Mailbox
Cardboard box
Magazine(s)

Basically, anything that will stabilize your camera works. Using the self-timer will lessen camera movement caused by you pressing the shutterbutton.

Personally, here’s my two favorite tripod substitutes:
1. Monopod – Easier to carry and move than a tripod. Excellent for sports photography or quick moving situations.
2. Me – I’ve learned how to hold my camera (right hand on camera body, left hand supporting lens, squeezing button smoothly) while bracing myself (one foot behind, turned, and the other foot in front facing forward). This way, I’ve been able to shoot camera-movementfree shots at very slow shutterspeeds.

Hope this helps.,