@d21lewis

http://www.forbes.com/forbes/2011/0228/technology-gabe-newell-videogames-valve-online-mayhem.html

This article's information is outdated by about 17 months but it has Half-Life 2 at 12 million (more popular than any Halo game ever released).

Now, we can infer that the retail sales for Half-Life since 2008 plus the Steam sales since 2004 combined with the 9.3 million copies it sold through retail (according to the gamesultra article) from launch to 2008 could total somewhere around 14-16 million copies. We could infer a similar total for Half-Life 2 if we combine Forbes' total with the last 11 months of retail plus lifetime Steam sales.

As for Counter-Strike, there was a GameSpot article posted on 12 August, 2011 that stated that the Counter-Strike series has sold 27 million copies. Again, outdated information and it conglomerates the entire series into one total. That being said, the majority of sales for CS are 1.6. Condition Zero was a total bomb and Source released four years after the original and really failed to surpass it thanks to the relative lack of modding. As of December 2008 the stand-alone, retail edition of Counter-Strikes sold 4 million copies (according to the gamesultra article).

http://upload.wikimedia.org/wikipedia/en/6/67/Counter-Strike_Box.jpg

This does not include sales through Steam or as part of the Half-Life Anthology or other bundles. Personally, I have a lot of friends who play(ed) CS 1.6 but I have only seen this edition once. I can not find the article but it was written in 2011 and it stated that CS 1.6 had sold a combined total of 15 million copies, Condition Zero 4 million and Counter-Strike: Source 8 million (these are rough approximations).

I wish I had more complete information but Valve has always obfuscated their digital sales numbers. Perhaps they feared that the success of Steam might attract competing digital distribution systems such as Origin.

@OP

Personally, I do not pine for the days before Call of Duty's virtual hedgemony over the FPS genre. The derivative nature of the genre has little to do with CoD's popularity or influence, but the general, industry wide trends that hurt innovation. Greatly increased development costs have stifled originality far more than CoD ever could. With the industry in the state that it currently is, it doesn't matter what FPS is on top. If Half-Life was on top today then developers would be playing it safe by copying that. If Halo was on top, the same would occur. If Counter-Strike was still on top then things would essentially be the same.

It is worth noting that despite many derivative FPS games out there, there are still many original experiences to be found. Finally, despite the fact that it has become more prevalent, the FPS genre has always been rife with uncreative, unoriginal clones. The secret has always been to simply avoid them in favor of the real innovators.

I was a big fan of Day of Defeat which was another Half-Life mod.  I never owned it but I played it a decent amount on a friend's computer during college days.  I have a few found memories from playing it online.  One was watching 2 teammates get shot up in a window then for some reason I was like fuck it and I just jumped through the window.  The 4-5 people that were on the other team must have all been reloading / not ready because they were quickly overrun and lost their last flag.  Sorry, I was just going down memory lane... 

I disagree with your popularity statement. Popularity does not mean sales. More people will know about Halo then Half life and counterstrike simply because of the media that Halo has spawned.

Halo is one of my favorite Shooter games I've ever played!

YES! It was best when Halo was King. (I despise CoD!!!)

Well, I most certainly disagree with your assertion that brand awareness is an important measure of popularity. Having millions more people playing CS 1.6, Half-Life and Half-Life 2 then even the best selling Halo is a clear indicator of which franchise is more popular. I may watch Red vs. Blue but I still play and enjoy Half-Life ten times as much as Halo. There are people who watch Red vs. Blue who never played a single match of Halo, how is that a good indicator of popularity? Additionally, consider the fact that we are discussing the period between the release of Half-Life and Modern Warfare (1998-2007). During this period Halo only achieved 6 million sales and Halo 2 8 million. If you combined these two titles you might have as many players as a single Valve game. Seriously, there is no comparison. Doom was king, then Half-Life and now Call of Duty.

I want to play Half Life 3