The swedish site
http://www.swecan.org/ offers a lot of information. It may not be totally objective information but it's a hell of lot better than the information the swedish government hands out.
On the subject of getting mentally ill of smoking cannabis I wouldn't believe that 'fact'. The human race have smoked/eaten cannabis for over 10 000 years and it's only recently that's it's become illegal for most parts of the world. It's probably true that some people have gone mental from smoking cannabis, but it could've just as easy been triggered by a number of other stimulants or emotional trauma. Read about latent psychosis.
What's important when debating legalisation of cannabis is to always make the con-debater present sources of his facts. ( I guess the same goes for the pro-debater, as well :-) ) If the source(s) is from a single-interest organization the facts are most often wrong. (Again, same goes for pro-side)
WHO - World health organization - Has presented a report where they compare alcohol and cannabis and the results were that cannabis, although it's not good for you, is way better than alcohol. A similar report can also be read from the canadian government I think.
When you debate, don't bring alcohol into the debate. Don't compare plague with cholera.
It's sometimes kinda funny when con-legalize side debates. They may say that if you smoke cannabis the dealers will try to lure you into heavier drugs. They want to make more money out of you, and you don't make much money selling cannabis. It's a silly argument since if you legalize it, the government will control the distribution. The same goes for saying that cannabis is a high-crime drug. Legalize it!
A drug-free society would really be great, but we're still humans and humans really want to use drugs in recreational purpose and that's never going to change.
If you want some history when cannabis got banned. You can read a little about it here
http://en.wikipedia.org/wiki/Harry_J._Anslinger
Harry J Anslinger was an evil man
His main argument for banning it was that if a black man smoked cannabis he would look TWICE on a white woman. He also added that it was only blacks, mexicans and homosexuals that smoked cannabis.
/F-Ray