Here are some for beginners.
One of my favorites to play is Flash Element TD. Its what got me hooked and is always fun to come back to.
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I wont embed any more games, but I will link some and also give links to pages devoted to tower defense games as well.
Desktop Tower Defense is a great game that doesn't give you boundaries by letting you place your tower anywhere on the map. They have a new game on Facebook that is called Desktop Defenders that you may find worth a try.
Bloons is a great beginner series hosted by Ninja Kiwi. They all have great animations and great graphics. The games aren't hard to master though and that is why they are in my beginner series.
These are more for moderate players.
The next 3 were developed by Pixelshock, they are good games but don't distinguish themselves to much from each other.
Armor Games Defense is a campaign and skirmish based TD that involves a bank that will give you a 5% increase of the deposits for every wave completed.
Pixelshock is a very basic TD game with no sound, and limited gameplay. Its good for a few hours then gets dull.
Pixelshock II however is extremely fun and entertaining. It is similar to Armor Games Defense but doesn't involve campaigns. It is only skirmishes and has a few various map modes to tweak your tower defense gaming.
The Next games are brought to you by Kongregate. I found myself playing these ones quite a bit after I got bored with my beginner TD games.
Cursed Treasure takes a new step in TD games by implementing towers that can only be built on certain terrains. This game is for you if your looking for a new form of strategy to play in your TD games.
Protector 4 is another strategy based TD game. In this one your towers can only level after they have a certain amount of experience from kills. This is another great TD game to play if your bored of the ordinary ones.
Hard Tower Defense Games.
These games I found to be a bit harder to master, some I haven't even beat yet. If your looking for a challenge these titles are for you.
Armor Games developed Gemcraft and Gemcraft Labyrinth. They are both great TD games that introduce an in-depth RPG feeling to the game by including a well thought out story line. The games have similarities but are by no means are the same game.
VectorTD is a great series published by CandyStand. This series is very innovative. The levels remind me of a virtual reality world that has no bounds. The games are all similar to each other but get harder as the series enhances itself. Check them all out, I am sure you won't be disappointed.
When and if you get bored of these, give Onslaught 2 a try. It has a massive amount of levels, the upgrades are phenomenal and the game is pure fun.