It depends on the mount, and on whether you can accept an optical element in the adapter.
Canon's EF mount has a very short register, which means a lot of lenses for other mounts can be adapted simply with an adapter that moves the lens a few millimeters away from the mount. However, with other mounts you need to get a lens element to achieve infinity focus. This element can affect optical performance.
The lenses that can be used with an adapter without an optical element include, but is not limited to:
- Nikon F mount
- Olympus OM
- Leica R mount
- M42 screwmount
Lenses that require an optical adapter include, but are not limited to:
- Canon FD mount (old Canon mount before EF)
- Minolta MD
Unless a lens is very unusual, or you're strapped for cash, it's often better to get a lens that doesn't require an optical adapter, as the choices are large for the Canon EF system.