If you'd like to send him a birthday message - www.happybirthdaymandela.com. If you wish him happy birthday you get a free download. (They're tracking the numbers, and Tanzania leads the pack in number of wishes! Go Tanzania!)
Read about the Aids Charity - www.46664.com - and donate in his name. (46664 was Nelson Mandela's prison number when he was imprisoned on Robben Island, off Cape Town in South Africa).
More info on www.SouthAfrica.info.
Here's an older video in which Johnny Clegg sings his Nelson Mandela tribute, and Nelson Mandela gets down...sort of. :-)