Yes, visas are required for Uganda, Kenya and Tanzania.
Most are available at the border however Kenya and Uganda are looking at revising this system and offering an online service.
For most nationalities the visa costs are:
Kenya - USD$50
Uganda - USD$50
Tanzania - USD$50 (note USA passport holders pay USD$100 for Tanzania)
Kenya and Uganda are in the process of changing over from visas being issued at point of entry to being available online.
South African’s staying in Kenya for 28 days or less don't require a visa. Those staying more than 28 days require a visa costing USD$50
It is handy to note Uganda, Kenya and Rwanda have introduced a joint visa called the East African Visa, this costs USD$100 and is available at point of entry and allows for travel throughout the 3 countries over 3 consecutive months.
For all other nationalities and up to date visa information please refer to Debs at firstname.lastname@example.org