Olympus, Nikon and Pentax lenses are easily adaptable to the EF mount with simple adapters that do not require optical correction to maintain infinity focus. See the complete list below....

Bob Atkins: Using Manual Focus Lenses on Canon EOS bodies

Canon EOS Lens Adapters

Flange to Focal Plane Distance

“The focus of a lens is determined by the distance from the lens to the sensor. A mechanical adapter which allows a lens to be mounted on an EOS body and focused to infinity is only possible if the lens is designed to focus an image at a distance greater than that between a standard EF series lens and the sensor in the EOS body. This is because you need some space for the mechanical adapter between the EOS body and the lens. If the lens is designed to focus an image at a shorter distance then an EOS EF lens, then the manual focus lens would have to be put inside the EOS body! The distance from the mounting flange on the back of the lens to the film (or digital sensor) is known as the "Flange Back", or the "Flange to Focal Plane" distance, or sometimes as the "Flange to Film" distance.”

”Here's a list of flange-to-focal-plane distances. For all Canon EOS cameras it is 44.0mm, which is shorter than most other major cameras. This enables (in theory) lens adapters to be constructed for the lenses shown in green below. Lenses shown in red (which includes all Canon FD and earlier lenses) cannot be mounted on an EOS body and still reach infinity focus without some intermediate optics.”