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Molniya orbits are popular with Russian communication satellites, because they spend a lot of time at high latitudes.
Europe and Canada are also quite far north. Do any European, American, or perhaps Canadian satellites use Molniya orbits? If yes, which ones are those? If not, why not?
The vast majority of Molniya satellites are Russian, but a bit of research found a few. Wikipedia has a list of NRO launches that include a few satellites in Molniya orbit. Being NRO, and classified, it's hard to verify this claim, but it doesn't surprise me. A list of all Earth Observing satellites popped up a NASA satellite as well.
Other countries seems to prefer constellations of LEO satellites for communication at high latitudes, usually in some sort of a polar or otherwise high inclination orbit. There is also a special orbit known as the Tundra orbit, where a Geosynchronous, but not stationary, satellite orbits in a particular ground pattern over a particular region. Sirius Satellite Radio is the only current users of this particular orbit.
I can't find TLEs, but here are two US satellites in Molniya orbits:
From the Southwest Research Institute's page for the TWINS mission:
TWINS flies as a mission of opportunity on two high-inclination, high altitude spacecraft provided by a non-NASA US government organization. Each spacecraft is 3-axis stabilized and approximately nadir pointing, and is placed in a Molniya orbit with 63.4° inclination and 7.2 RE apogee, an ideal orbit for magnetospheric imaging. TWINS began its stereo imaging mission in June 2008.
The Wikipedia page for TWINS says:
Using
$$T = 2 \pi \sqrt{a^3 / GM_E}$$
from this answer with $GM_E\approx$ 3.986E+14 m^3/s^2 one can verify these have periods of a half-sidereal day.
