Some of our light switches are giving light shocks and sometimes you can see a spark. What
Could be static - kinda depends on the nature of the shock/spark. Not uncommon in the winter (dry inside the house) to get a static discharge to the grounded screws - try touching a doorknob or some other metal thing before touching the switch - if the other metal thing has the shock then, and the switch does not, that might be it. If you still get a shock from the switch, move on to:
If it's more than just static, call an electrician ASAP...
As others have said, it's most likely just static-shock. In my experience, this happens a lot in houses in which the switches aren't grounded.
The solution would be to add a ground-wire to ground the switches, but to do that you typically need to replace the wiring for the entire circuit. Given that this is a relatively minor problem, and the solution is very time- and work-intensive, it's probably not worth it (though, adding a ground does have other safety-benefits...)