I'd recommend Defense Grid: The Awakening ^[1]

I love Plants Vs Zombies ^[1]. It's like tower defense but with a fun spin on it.

I can't get enough of Gemcraft chapter 0 ^[1].

It also has achievements, which I haven't found in very many flash-based tower defense games.

I find Bloons Tower Defense 4 ^[1] extremely fun. Also, Desktop Tower Defense ^[2] is a classic. (Both of these are flash games.)

Sol Survivor ^[1]

Sol Survivor is a tower defense in the "side" model (no maze, you build on side on the defined roads only).

It adds a nice feature to the genre, "support" skills, that you can activate to attack/slow down/poison/scare the enemies. Thanks to that, you don't have to watch desperately a unit your towers miss go to th end, you can catch it after. It also gives you something to do, instead of only watching your towers do the job.

It features several characters, each with their own set of turrets and support skills, so each of them plays differently. This is particularly good when you play in cooperation with friends, as your turrets and support are completing eachother.

I personally like Desktop Tower Defense ^[1]. The fun modes are interesting.

ccook adds: There is also a facebook version which has some more features such as unlocking towers.

Cursed Treasure ^[1]: Don't Touch My Gems - a flash based TD Unlike in other TDs, where you protect your Castle, the Goal is to protect your Gems and don't let the monsters steal them - like the subtitle says even don't touch them for Gold Rating on each map.

• 15 Maps with different difficulties
• 3 different towers each upgradable, can be spezialized in 2 ways
• 3 spells to support your towers
• many different enemies with special attributes
• level up your character to increase the efficiency of towers and skills

Even though Tower Defense games have grown into their own interesting and innovative genre, I still prefer Warcraft 3 Tower Defense maps.

There are a whole bunch of Tower Defense variants ^[1] at Kongregate ^[2]. One of my favorites was Whiteboard Tower Defense ^[3].

Be careful, I've burned many an hour there. ;)

I really like the Protector series, with the best Protector IV ^[1]. Extended storyline, hours of play. Extensive strategy behind the upgrades system (experience, items, etc.) and the units you select for each quest. Multiple storylines and sidequests, dozens of units.

Bubble Tanks ^[1] is really fun and offers a nice variety of maps. You can also merge your towers into super towers which adds to the strategy.

This is actually from the upgraded version ^[2]. They added some new tower types like the beam canon featured in the center. There are also some new creeps which knock out some of your towers for a short period when they die (thus, the grayed out towers at the front).

I love vertor td ^[1] and vercor td 2 ^[2]. Both are easy to start with but they get pretty hard very soon. But i still prefer the starcraft and warcraft 3 mods.

Defense Grid ^[1] is excellent, it's from an independent publisher, with very high re-playability, and only costs $10 on steam.

I highly recommend it.

There is a really nice flash based Starcraft 2 Tower Defense game

http://starcraft2.pro/tower-defense

There is a Custom Map in Starcraft II that seems quite popular (and fun)

http://www.sc2mapster.com/maps/standard-tower-defense/

http://www.youtube.com/watch?v=4Io0ryOPrxM

Try the "Bloons Tower Defense" series, very addicting.

Onslaught 2 ^[1] adds some cool new ideas like tower combos. Build two towers next to each other and fully upgrade them and they will work together in a combo.

Harvest Tower Defense ^[1] has a really interesting and unique energy management grid mechanic that made it really fun.

A very old one: The Horde ^[1]

In the game you develop a village and prepare for the Horde onslaught. Trick is to balance development and defenses through seasons.

It's abandonware now and available for download. It works with Dosbox.