As a program manager for household hazardous waste (HHW) I get stuck with alot of waste latex (water-based) paints.  So I began a campaign; Use it Up, Pass it on, or Dry it up! The best thing to do with it is to use it up!  Apply another coat to a shed or doghouse, mix waste latex paints together and use it for primer.
Pass it on! Also, if someone else is in need of paint, donate it to them. Schools community theaters are always looking for paint.  If all else fails, Dry it up! Put some cat-litter in it to solidify it and chuck in in the trash. Of course, you should always buy just enough for the job. While paint recycling is feasible it really turns out to be a budget buster for HHW programs unless the reclaimation factory is nearby.
Oil or solvent-based paints are always hazardous and need to come to an HHW program for proper disposal.  Also, they don't solidify the way latex does.