(Not in Any order)

1. Insurgency

http://www.insmod.net/

Great tactical gameplay and it is very very polished for a source mod. It's trying to be realistic so if you don't like one/two shot kill games then you wont enjoy Insurgency. It's also a very team based gameplay where one team assaults and captures points and has to try and capture them all. Theres different kinds of gameplay depending on different maps and theres lots of people playing and it is on Steam so if you search for it then you can download it there for free.

2. MINERVA

http://www.moddb.com/mods/minerva/downloads/minerva-metastasis-full-patched

An amazing single player game based on the Source engine and I believe Valve actually hired the guy who made this because of the fantastic level design. Everyone I know whose played it really really loves it... just aslong as they like the gameplay in HL2.

3. SourceForts

http://www.sourcefortsmod.com/

Just a really fun Team Death Match game where you have a build time to build up your fort to try and protect your flag. Each team has classes playing different roles and theres an Eng class which trys to break down your fort and you have to defend it to stop them doing that and getting to your flag, aswel as attacking. Theres not alot to it and it uses all the HL2 resources but it is amazingly fun!

4. Weekday Warrior

http://students.guildhall.smu.edu/~weekdaywarrior/

Amazingly funny old skool style of adventure game. It's not point and click but you will be solving puzzles and all that to try and work out whats going on at the the work place. It's not annoying and doesn't have lots of item collection, just lots of fun mini game type things as your progress through the story.

5. Half-Life 2: Wars

http://www.moddb.com/mods/half-life-2-wars

An amazing Single Player RTS but they are planning on adding multiplayer. It's very short right now but it is amazingly polished and has a great UI and is really fun. I love all the physics in the game and the path finding is really good, almost feels like a professionally made HL2 RTS. Just needs more levels and a multiplayer, but they are still working on it.

6. Eternal Silence

http://www.moddb.com/mods/eternal-silence

Another mod you can download straight from Steam for Free. This is another amazingly polished Online FPS that mixes assault team based gameplay along with space ship combat. It's very much like Star Wars Battlefront where you take off from the ship and fly about in space shooting eachother and then landing on the enemy ship to try and achieve the objective.

7. Fortress Forever

http://www.fortress-forever.com/

Supported on Steam again this is another amazingly polished Online FPS that is basically TFC on Source. If you didn't like TF2 over TFC or you want a TF2 experience but can't afford it then this is a mod you'll love. While TF2 and FF/TFC have similar gameplay mechanics they do play very differently.

8. Portal: Prelude

http://www.portalprelude.com/

You do need Portal to play this but it is so good it feels like it could have been made by the developers. It has great writing and voice acting aswel which I find rare for mods and it is quite funny too. This feels like a nature progression to Portal because if you found that game too easy then this one is MUCH MUCH harder!

9. Zombie Panic: Source

http://www.moddb.com/mods/zombie-panic-source

Supported by Steam like some of the others this is another online FPS zombie game. One person on the server is chosen to be the starting zombie and everyone else is humans and basically the goal of the zombies is to infect all the humans and the humans objective is to survive as long as possible. The fun of this game really depends on the maps though so you will have to spend awhile looking for good servers.

10. The Hidden Source

http://www.hidden-source.com/

Another online FPS where one guy plays The Hidden whose invisible and the rest of the server has to try and find him and hunt him down. The Hidden only has melee and has to sneak up and kill each person one by one. The players hunting the Hidden are given tools to try and find him like trip alarms or laser sights etc etc.

Bonus amazing mod that isn't free anymore:

11. Garry's Mod

http://store.steampowered.com/app/4000/

This is my personal fave mod that is only £5 but it offers so much value because youc an play any of Valve's Source games with it and enhance them. It gives you the tools to play around with the physics and make contraptions like your own vehicles out of objects using physics tools. Theres all kinds of servers from Zombie Panic type of gameplay to build servers to Role Playing servers. My personal fave thing to do though is open up Half-Life 2 in Garry's Mod and play through the campaign and just messing about with the AI or building my own vehicles and just doing crazy things. It's also the tool people use to create Machinima like War of the Servers which is an amazing player created movie...

http://litfusefilms.com/movies/waroftheservers/

Or EVE Online's fantastic Clear Skies movie which is done in Garry's Mod.

http://www.eveonline.com/ingameboard.asp?a=topic&threadID=783871

All these mods are really great and you can find even more over at:

http://www.moddb.com/

Enjoy.