I have just realised that my hard drives are getting pretty hot (I have 2 of them on on top of the other, and tow CD-Rom are at the bottom)

I do not have a case. they are all expose in air.

Are there any fans for harddisk??

Well, you don't have a case. Are the drvies external drives or internal drives?
I don't know of any cooling fans for external drives but there are cooling fans for internal drives. http://www.antec.com/ should have what you need.

Aditionally, if you aren't confined to a case, you could simply rig a standard case cooling fan to blow between the drives but you'll need to leave room between the drives.

Stay cool,
vujsa

Yea...I don't have a case.
They are all internal hard drives.

I may use a normal fans to cool them down..

Either....

1) buy a case, if you find a good shop theu are dirt cheap.
2) make a case yourself
3) dont stack the drives... buy a round ide cable and ut them side by side
4) get some rubber feet, or make wedges so seperate the stacked drives, put some air between the disks.

You really should have a case, a case will Protect your computer from possable sources of interfereance, and keep the dusk out.

buying a case would be a very good idea especially since they are internal hardrives.

that way there can be fans to circulate the hot air and you might be able to get fans to directly cool down the hardrive. it will also protect the hardrive from damage.