At Target a few months ago I found a little 12V electric
warmer made by Vector.  It was about $12 on close-out.
It looks basically like a soft-sided zippered lunch box with a
cord coming out of it.  It folds flat.  When plugged in, it heats
to about 200 degrees - not enough for cooking, but enough
to warm up rolls, or baby food, or other things that are hard
to put on the stove.  It's not fast, but it's very portable, and we've
gotten more use out of it than we expected.

                                              -Steven Sittser