Typing this on my MacBook with OS 10.4.11. I principally use ExaChess (full version), but have also used SCID. SigmaChess is a lower priced alternative (I have it too), but has some drawbacks. A couple of comments.
Exachess is very flexible, and does things that SCID won’t, such as open multiple game windows, and search across multiple databases. Still on version 3, which does have some bugs, but has perfectly good work arounds for all the ones I’m aware of. I had a beta version of ExaChess 4 from the author late last year, but haven’t heard anything since. It’s inconvenient to order; I ended up sending a cashier’s check to Australia, as he isn’t set up with PayPal. Biggest disadvantage, although you can search across multiple databases with almost any number of total games, it has problems with individual databases starting somewhere around four or five hundred thousand. When I bought a 3 million game megabase from Chess Assistant, I had to use SCID to break it down into 10 PGN databases which Exachess could convert to its own format.
SCID. If you have better Unix skills than me, you may be able to figure out how to use the very latest version, 3.6.xx. I’m using version 3.5, which is the latest version provided for use with Fink. With this version, at least, you can only search one database at a time, and can’t open multiple game windows. PGN databases open as read only; you need to save to SCID format to edit anything. It’s a bit quirkier and less user friendly than ExaChess, IMHO. On the other hand, it will handle large databases, searches are fast, and it has a lot of options for annotations, generating opening reports, and other useful functions. Considering that it’s free, there is absolutely no reason not to try it. You’ll need to install X11 as well.
SigmaChess is a great front end for engines like HIARCS and Fruit. It will handle databases of up to a million games (but no more), and the full version is quite inexpensive. It will not search across multiple databases, and the search mask is less sophisticated than SCID or ExaChess. The worst disadvantage is that alternate lines can only be saved as text comments. That is, you can’t play through a variation by clicking on the moves the way you can with the other programs.
I guess I should give some links:
exachess.com/
sigmachess.com/home.html
finkproject.org/ (If you want to install SCID, and will settle for version 3.5, then install Fink, and follow the instructions on the Fink page for installing SCID, or any other supported program that you want)