Hey, 
 
I just finished Portal 2 Coop with a friend and now we intend to go back to the beginnings of the Half Life Universe. We would like to play trough 
Half Life Source and Half Life 2 cooperatively but we are not sure if there are well working coop mods out there. 
I found this one for Half Life Source: http://www.hlportal.de/?site=files&do=showfile&file_id=325 
 
I couldn't check it out myself since I don't own HL Source, yet, so it would be cool if you could share your experiences with that mod (or any other working 
mod) with the community.  
 
There is this Mod called Synergy for Half Life 2 which seems to be a little more advanced but I still have to check that out as well: http://synergymod.net/Media 
 
Anyway, thanks for your comments/thoughts/advises 

The only ones I really know of are Synergy, which you have already stated, Obsidian Conflict and I believe Garry's Mod has a way to playing coop. But to be honest I am not sure that any of them were specifically made, or have the ability to play Half-Life: Source. Most of them seem to focus on HL2 or custom coop maps.
 
Although, to be completely honest. If you have the original Half-Life or can spend $10.00 to buy it, the Sven Coop mod is almost absolutely your best way to enjoy co-op play with the original Half-Life. Also, if you are feeling up to it, you could purchase the Half-Life 1 Anthology for $15.00 on steam and I believe Sven Coop supports Opposing Force and Blue shift as well. Not only that, but Sven Coop has mad amounts of really fun user created Half-Life style of coop maps. I know your wanting HL:S, but considering it's a pretty shitty port, I would really go with Sven Coop.
 
Hope this helps. :)

The only ones I really know of are Synergy, which you have already stated, Obsidian Conflict and I believe Garry's Mod has a way to playing coop. But to be honest I am not sure that any of them were specifically made, or have the ability to play Half-Life: Source. Most of them seem to focus on HL2 or custom coop maps.  Although, to be completely honest. If you have the original Half-Life or can spend $10.00 to buy it, the Sven Coop is almost absolutely your best way to enjoy co-op play with the original Half-Life. Also, if you are feeling up to it, you could purchase the Half-Life 1 Anthology for $15.00 on steam and I believe Sven Coop supports Opposing Force and Blue shift as well. Not only that, but Sven Coop has mad amounts of really fun user created Half-Life style of coop maps. I know your wanting HL:S, but considering it's a pretty shitty port, I would really go with Sven Coop.  Hope this helps. :)

The mod I posted for Half Life Source seems to be dead: http://halflifelibrary.com/wiki/Half-Life:_Source_Coop 
 
@Errordotocx: I didn't know that Sven Coop works with the regular single player. I played it when it was released... a decade ago or so and I think it only featured user generated levels. Thanks for the tip!