This may sound silly but Yugi-oh has a few figures that work great as monster minis
The pumpkin king makes for a decent beholder.
They also have several really cool dragons.
Steve Jackson Games' cardboard heroes are awesome. You cut them out and they have a front side and a back side to the figure and you fold them into a triangle shape, and they make very good miniatures that travel easily. I wish he would make a larger variety.
The Fiery Dragon counters are also fantastic, I just wish they would make them like the Steve Jackson cardboard heroes to where they have a front and a back and they stood up instead of lay flat.
I have on occasion copied art from the WotC galleries and made my own little counters.
If you take a jpeg, cut and paste it into a word document, then you can right click on the picture and select "format picture", click on the size tab to reduce or enlarge it to the right number of inches (or squares). Then you can flip and rotate the image and paste another copy right beneath it so that you have two images touching at the head (or the top).
Then just print and cut out your figure, fold it in half and you have a sandwich board mini that will stand up. You could tape it to a penny or a quarter to make it stand better and keep from blowing around if you like, but you don't need to.
I have also on occasion drawn my own minis. One time I needed a zombie camel and a zombie minotaur, so I just drew my own, large size, then I shrunk them down on the copier at work to the right size for a mini, and they worked great.
I uses minis every game, it helps tremendously. I like the minis to look like the monster or the character they represent, I think it adds to the game quite a bit.