I've been working my way through SWAT 4 recently and am loving it. The slow pace and methodical gameplay is refreshing; it feels very authentic and very tense. I've always been interested in the tactical shooter genre, but aside from the Red Orchestra series (where I could never really find people to talk to and coordinate with anyway), I haven't really played any. SWAT 4 is the first one I've really sunk my teeth into.

I was wondering what other good, realistic tactical shooters are out there that some of you could recommend. Preferably single-player? Seems the genre's sort of died, based on my googling at least. The new Rainbow Six games for example don't seem to be as tactically deep as the older ones. I'm open to new or old though.

Well Frozen Synamps is at the one extreme of the "tactical shooter" spectrum, though probably not what you are after.

The Rainbow Six: Vegas games are excellent and should provide you with what you're looking for.

I've only played Rainbow Six: Rogue Spear, SWAT3 and SWAT4 and while I enjoyed them all, SWAT4 is my favorite.

And as you say, the newer Rainbow Six games like Rainbow Six: Vegas aren't really that tactical compared to the games mentioned above.

And even though the genre pretty much been dead for a decade now, there are some bright spots with these kickstarters.
http://www.kickstarter.com/projects/355932838/crowdsourced-hardcoe-tactical-shooter
http://www.kickstarter.com/projects/670743543/ground-branch (unfortunately failed, but they are still going, slowly)
http://www.kickstarter.com/projects/1687497632/h-hour-worlds-elite-0 (mainly multiplayer focus but a single player campaign will come eventually they say)

In any case, the very first Rainbow Six and SWAT3 are available on http://www.gog.com/ so you can give them a try.

Ahh Rogue Spear, how I miss that game.

The OP did mention them as not as tactically deep as the old ones, and I agree with him about that, but I am also with you that they are excellent games.

EDIT: Also, the quote in your sig freaks me out. That is all.