I just use sheets of paper.
I have a ton of skeleton key games dungeon rooms printed out on paper, I just print them, cut them out, set them down room, by room as needed. They are totally reusable.
I take a lot of the maps from Dungeon magazine, scale them up to 1" = 5' and then print out each room separately. If secret doors are marked, I just erase them with the blur tool in the picture editor that came on my computer.
Also, using just MS word, I can take any picture from the net, lay a grid down over it (just make a 8" by 10" table, with 1" squares) and print it out. I can make a battle mat out of any picture I find on the net. Use google image search to find any kind of floorplan or terrain you want.
I also just made a lot of blank grids, with different looking surfaces: marble, cobblestone, wood planks, pebble, sand, grass, stone. I have a stack of these handy, and as needed I can freehand draw a room and lay it out, or just cut out the shape of the room with scisors.
Honestly, these battlemats are great for our game. They add a lot to the feel. They scroll, in that I only lay down the rooms as necessary, the party can't see what's coming next. They don't need much space, so you can play on a coffee table or even a surface the size of a side table or a spare kitchen chair. They are cheap and portable, reusable, disposable, and you can write on them.
Paper battlemats are cool!