My current town is grossly overpopulated. Is there any way to get sims to leave town or otherwise lower the population?

Do you have Twallan's MasterController? I believe there's a setting, you can set how many Sims can be in your world. I think that limiting how many houses you have can help, because lots of times if I'm playing University I get notices that more Sims would move in but there needs to be more housing first.

First of all, disable EA Story Progression - it is useless and screwed up. Then, use Twallan's Register to disable immigration and tourists (as well as some or all pets - I never have stray horses in my towns, because they look really out of place in most worlds). If you do want a story progression, take a look at Story Progression mod at N Raas (and read instructions to set the settings to your liking).
Use N Raas Overwatch to delete your homeless sims (if you had EA SP on, you probably have tons of them - much more than your town needs to occupy NPC careers).
If you have apartment buildings in your town, do not set neighbors' apartment doors as NPC doors, or at least not all of them - maybe, you get many extra sims generated because of those doors.

You can connect some unpopulated or underpopulated worlds to your current town via N Raas Traveler, and have some of your playables emigrate to those worlds.
Also, you can release your inner sadist and kill off some of your playable and unplayable population in various ways, so that you have less sims, but more ghosts to haunt your cemetery or other places at night, hehe.

I have NRaas story progression, master controller, overwatch and traveler, but I just let them do their thing. I'll play around with traveler and see about connecting worlds. Otherwise I'll see about killing off some sims or getting the Population module for story progression.

I believe you can use Nraas Register to cap off the population of wild animals and Sims, it's on a town-by-town basis.
If you do cap it off, you will have to disable immigration in both Register and Story Progression.

As for lowering the population, you can always Force Kill a few Sims or wait until the elders die off.

I've been having a lot of elders dying off lately, as the oldest of gen 3 just became a teenager. I've also noticed there's been some elders being saved by their pets.

Master Controller has the "Force Kill" option. And I typically set the Elder stage to 2 days, with a maximum of 7 days beyond that, though on a few individual Sims I turn that limitation off.

I never knew you could adjust how many kids a household can have. It's at whatever the default is and I'll see about turning it down!

LOTS OF FIRE

Or, for a more clean Logan's Run genocide thing, NRAAS/Twallan mods. Wipe 'em out, then configure population caps.

That's the base number of children a sim will want to have, not the number they are allowed to have.

I suggest that the OP continue the discussion at NRaas. They have already opened a thread there about emigration, to which I have responded, but it's on a sub-board that doesn't get much traffic. This one would be more visible to more of our members.
http://nraas.wikispaces.com/page/messages/Chatterbox

Thank you for your replies! I made a post in the chatterbox on the nraas site.