Two that I use (these are not recommendations, just info):

1) I use a small 1300W cube heater to take the chill off of my 3-stall
garage workshop. Obviously this is limited on capability but will bring the
temp up on a chilly day (45 degrees).

2) During cold weather I use a 23,000 btu kerosene heater. This is used
with a fair amount of caution. Cars and all other gasoline are removed from
the garage. It resides in stall #1 while most work is done in #3 and #2.
Also keep the floor and the heater itself clean. This does smell a bit
during startup and shutdown but otherwise ok. I often supplement with the
cube heater and if conditions are not too severe I can heat the garage with
kero and then turn the flame out and sustain with the cube. Downer -
Kerosene is getting expensive.

3) A previous shop was heated with a recycled forced air furnace. Paid $25
for the furnace. Probably another couple of hundred to hook it up. It was
a down-draft suspended about 1' above the floor on a simple metal frame that
allowed heat to blow down onto the floor of the small shop. Larger shop
would have benefitted from ductwork. Another reason to raise the furnace is
to get the firebox 3-4' above the floor and away from fumes and dust
accumulation. Still needs frequent cleaning.

RonB