Are you looking for just an engine, or an engine plus a graphical user interface? I'm not a Mac user, so I'm in unfamiliar territory. If I were you, I'd try to set something up that uses endgame tablebases. I think that would help with your endgame analysis.
Some free possibilities:
I think the Scid interface (and also Scid vs. PC) has an install for Macs, and it can handle Nalimov tablebases.
For engines, if you won't be using the tablebases, I'd try Stockfish. It doesn't use tablebases, but it's pretty good in the endgame. (Maybe try it anyway, even if you plan on using tablebases. You might like the engine.) If you'll be using the tablebases, then I'd try Fruit 2.3.1. It's not quite as strong as Stockfish, but it can handle Nalimov tablebases.
http://scidvspc.sourceforge.net/index.html
http://www.stockfishchess.com/
http://www.superchessengine.com/fruit.htm
You can find the 3-4-5 piece Nalimov tablebases here (download the 3, 4, AND 5 piece files):
http://folk.uib.no/pfvaf/chesslib/Nalimov.htm
I've been using Chess Tiger to examine a lot of endgame possibilities (edit to add: no, not games in progress, just things that occur to me, and a few positions that I've been in at my club then wanted to know more about when I get home), but find that its positional calculations aren't perfect when they could be.
For example:
Now, with best play this position is clearly drawn, as White cannot get the pawn promoted unless Black plays suboptimally.
Yet, the best line calculation on Chess Tiger reads:
D55 +0.33 Kf3 Kf6 Kf4 Kg6 g5 Kg7...
Now, with best play, it's going to be a forced draw in 6 moves. So why is it calculating 55 moves deep and telling me that White has an advantage of +0.33?
Interestingly, playing through the six moves until the forced draw, it tells me on some moves that the positional score is 0.00, on some moves that it's +0.20, and some moves that it's +0.33; all with the same best line calculation.
Of course I'll still grant its 55-59 move deep calculations, because if it doesn't calculate suboptimal lines then it won't know what the best lines are, but still, the above issue stands.
Assuming that the Dn refers to that it is calculating n moves deep, which is what I've inferred from the behaviour of the number in that field, but Chess Tiger comes with no instructions or explanation, so it's just an educated guess.
So... Looking for a program that will tell me as many moves in advance as it knows where the line(s) that it's calculating will *actually* lead.
Now, while I'd like the option to play things through to the finish to see how it's done, I'd like something that (akin to it saying M7+ or M7- for a forced mate in so many moves) will tell me when there is a forced draw in so many moves, too.
Speaking of forced mate calculations, Chess Tiger often messes up that, too. Sometimes I'll examine a position and it'll say there is a mate in so many moves, I'll look further down its best line calculation, and a move later it will tell me that mate is impending in the same number of moves that it said it was the move previously.
So, while Chess Tiger is fun to play against, because it plays better than I do, it rather sucks for endgame analysis.
So... I see that Rybka requires a lot of jumping through hoops to not quite make it work on a Mac, Fritz is available to work on a Mac, but I'll look for opinions before spending the money on it if there's something equal or better available for less or even for free, especially as I don't know for sure that the above engines have the features I'm looking for.
Suggestions? Pointers?
PS, I do think it'd be a great addition to the site if there would be an engine available for positional analysis (analysing any position, not just the (useful in its own way) analysis tool already provided for analysing whole games that have just been played). As for the effect this would have on cheating, if anything it'd wipe out a few cheaters: non cheaters aren't going to cheat anyway, and cheaters will cheat regardless, but if any cheaters are stupid enough to use the site's own analysis engine to cheat, they'll be more easily caught than if they were using any of the many available engines elsewhere.
Not sure about the legal and / or technical challenges associated with the addition of such a feature, though.
PPS, while I'm looking for a strong Mac-compatible engine with the above features, if anyone reading this has a burning desire to tell me that instead of approaching endgame study in this way I should do such-and-such, then by all means feel free to go ahead and spill; I'll be glad to take a look at your suggestion(s) too.