Mathlearningcenter has a nice and simple site to use - Number Rack.
The site starts with two racks having 5 red beads and 5 white beads which allow you to slide them back and forth. This is really helpful as you can show different ways to represent addition and subtraction problems.
You can also had up to 10 total racks (racks 6-10 have the reverse colors of the beads).
Embedded below is a 2 minute and 20 second YouTube tutorial.