I tend to watch a lot of sci-fi (television shows and theatrical movies), and this is reflected in my list of favorite character actors.

In no particular order:

Christopher Lloyd, Vincent Schivelli, Tony Todd, Jeff Kober, and the late great Ian Wolfe. These guys are not exactly household names (with the possible exception of Lloyd), but all of them have done great work.

As far as the old westerns (I'm a big fan of these as well) are concerned, Royal Dano and James Best are at the top of my list.