How can I take horizontal photos where the object is lying flat on the floor? Do you know any stands/tripods which allow to point the camera straight downwards?
The Manfrotto 55XPROB has a center column that swings over the legs and allows to shoot straight down. Thats the most common one that I'm aware of. Its a pretty solid tripod to boot.
EDIT: I just checked another tripod I have (a Dolica Proline 62"), which I think is the best-bang-for-your-buck at $40, its pretty stable, independent legs, and a ball head included. It ALSO can remove the center post, flip it, and position the camera between the legs instead of above them.
The Slik Sprint Pro does it too. You get to reverse the center column so that the camera gets attached between the legs and it becomes easy to shoot downwards.
It paid $120 USD for it and sold it last week for $60.
As many have stated already, there are tripods that allow you to swing the center column. With an inclinable center column you are able to adjust the camera to point directly towards the floor. Keep in mind that statics is pushed to the max, when you tilt the center column to 90 degree and extend it completely. Therefore I would not try to save too much on the tripod, so you probably may want to shop for a tripod that supports heavier gear that you actually want to put on.
To align your camera absolutely horizontal to the ground you might want to use the spirit level on your tripod or head. If your tripod nor your head has a spirit level you can buy spirit levels that fit into the flash shoe of your camera.
I recently bought a new tripod that I really like and is capable to do what you are looking for. It is the Vanguard Alta Pro 284 CT (B&H Shop, review) with a Manfrotto 054 Magnesium Ball Head Q5. The cool thing about this tripods' center column is that you can set it up from zero to 180-degree angle. However, it has a couple of other features that make a good price/performance ratio in my opinion. With this combination of tripod and head nearly any angle and position is feasible.
I use a simple inexpensive device called the ALZO HORIZONTAL CAMERA MOUNT. I found it here http://www.alzodigital.com/online_store/Horiz_camera_mount.htm
I was able to find a Manfrotto 3021BPro tripod used for around $80. Problem that you will need a head too (I ended up buying the Manfrotto 322CR2 head for $120). Like its successor, the 55XProB, its center column can be reversed, or fixed horizontally.
Regarding the suggestion by @ElendilTheTall, the proposed method is perfectly valid, and was used a lot before I got me my current tripod. You can tilt the setup as much as you need, counterbalancing the camera's weight with hanging something heavy on the opposite leg. For getting the legs out of sight, play with longer focal lengths.