open mike and jam are sometimes used interchangeably but they can be very different too.
I think there's one of these for every metro area:
Open Mic Baltimore
I guessed at Baltimore, but search for your area...
Here's another one:
Open Mics in Baltimore, MD (openmikes.org)
or this one, if you prefer Facebook contacts:
Open Mic Baltimore
There are a lot of open mikes that are genre-specific. In Philadelphia, I can find a blues event any night of the week. There's another 3-4 jazz.
And some are, as you noted, aimed at singer-songwriter solo or duo acts. And some cater to comedy... Some let you do almost anything for 15 minutes a night...
"Often" a 'jam' is more highly structured, and almost always genre-specific.
I usually advise checking out an event before committing to it; quality and tolerance of the host can vary greatly. *Some* can be very cliquish, and if you're not in the host guy's inner circle, it can be unpleasant. Some have the same 6 guys show up every week and play the same 3 songs. Sometimes the house band will hog the stage for a 1-hr 'opening' set and a 30-minute closer - and everybody else gets rushed in between, as if they're doing you a huge favor.
But MOST, and almost all blues events I have attended, have been very accommodating and welcoming.