Yes, REAPER comes with three built-in Virtual instruments:
ReaSamplOmatic5000 is a quite powerful sampler that has multi-velocity and multi-layer capabilities.
ReaSynth is a very simple additive synthesizer.
ReaSynthDr is an extremely bare-bones, 4-piece drum sampler that can be set on one track or as a multi-out instrument.
You can also browse through the ReaPack repositories and find a few interesting and powerful JS FX instruments like the Super8 MIDI looper.