Basilisk II is an Open Source 68k Macintosh emulator.

Release notes from J.F.:

Here’s test 18. Cross your fingers and hope this fixes it ONCE AND FOR ALL. If you’re any decent at beta testing, you’ll notice the new PSP setting for relaxed timing on the 60 Hz IRQ. When enabled, the 60 Hz timer waits about 16 ms from whatever the time currently is, so you get the 60 Hz IRQ about 60 times a sec, but no guarantees. Disabled uses the default B2 code to try to maintain a drift-free 60 Hz interrupt. This naturally uses more time and can bog down the Mac depending on what else is going on. The default is relaxed timing, which is much smoother.