There are tons of games online that you can play for free. Some are decent and some are good but a small amount of them have great programming and a great concept. Most of these games can be found on newgrounds. Some of the entries are a series that are better as a whole then just one game.
#5 Exit Path
Exit path is a great game by an amazing programmer named jmtb02. There are two in the series but I prefer the original. You are a stick figure who has to run through obsticles to your freedom and has to escape an evil facility. It is a great game that requires good reaction time and overall skill.
#4 Karoshi Suicide Salaryman
This one is very interesting it is based after the japanese word karoshi that means "Death From Overwork." The main goal is to die. Some levels are easy and some levels require split second timing. It is different from other games and is always fun to play.
#3 Interactive Buddy
Interactive buddy is just like interacting with a normal buddy, if you beat up your buddy and make waves of babies to attack him. This game is great and you can do amazing things when you get the interactive script. You can make fountains or the previously mentioned baby wave. You can play this to blow off steam and it has a great system with very few glitches and lag.
#2 Achievement Unlocked
Described as "metagaming hell" on the website this game isn't about levels. The only goal is to unlock acheivments. This is also a series and is very fun. The acheivment names get you thinking and some are very hard to figure out. It is very fun and addictive and is also made by jmtb02 who is one of my favorite programmers.
#1 Madness series
While this game may be more than a little violent I just love it. It starts out with our hero hank somewhere in Nevada. There are more videos in the series than there are games and a lot of the games are not made by the original creator krinkles they are still high quality games. My favorite game is Madness interactive the most popular one is Madess Project Nexus.
You can play these on Newgrounds and other websites. I highly recommend playing all of them.