Can I use manual flashguns with Nikon D5000?

I have a Nikon D5000 and also got few old manual / auto flashguns from different manufacturers. Is it possible for me to use these flashguns with the DSLR. I do not mind loosing the automatic functions.

You can but check the trigger voltage to keep you from blowing up your camera. Here's a list http://www.botzilla.com/photo/strobeVolts.html and instructions on how to measure the voltage if it's not on the list.

Yes as long as the trigger voltage on the flash is below about 9V, so check or test it to be sure;
http://www.botzilla.com/photo/strobeVolts.html

As you accept, you will not have TTL flash metering, so you need a flash that has full manual control of flash output.

On camera flash is not flattering - consider investing in some off camera radio triggers (RF602's are about £25 a set) - this will allow directionality & modifying the flash. If your flashes turn out to be unsuitable (trigger voltage too high) I can recommend the Yongnuo YN560 flash - about the same spec as a Canon 580 speedlight, but costs around £60.It's a modern digital flash, so no worries about voltage either and is fully manually adjustable. Can also act as a slave, so can be triggered by other flashes (as well as the RF602's I mentioned).