There's also a load of Various Indie Bundles out there, with varying degrees of Mac support inside.
The Original one, Humble Bundle, works the hardest to give Win/Mac/Linux on their sales (though they have been letting that slip lately). I do believe they have their own developers on staff to help make those ports possible, Humble is definitely one of the good guys here. Humble also has a
"Store", though they don't have a unified store front. Humble provides a widget for developers to throw on their own website + a unified download experience for the customer. Again, their usual MO is to offer all platforms DRM free + a Steam key, read all the details before buying.
There's also Indie Royale, Groupees, IndieGamestand (1 game every 4 days, BTA is usually $2). Desura (Indiegala) is another Steam/Origin-alike, I can never really tell what they're doing for downloading or using their client. GamersGate can have some great sales, but they are a bit of a mess about listing accurate DRM info, or keeping up with patch releases. GOG is the only one that really sticks to their DRM-free guns, now offering classic Windows/DOS games in a Mac wrapper alongside new indie titles. I slightly prefer Humble to GOG for throwing in Steam keys with the deal, they're both worth checking up on frequently.
Wineskin is a tool to wrap up Windows games to look like a regular Mac.app, and what GOG uses for any of their classic game releases. You can use it yourself to wrap up many games, look up a game at WineHQ to gauge your odds of success.
If you have the option to get a Mac game with the PC version thrown in, go for that instead of only the Mac build. Some Mac ports are just awful (ahem, Super Meat Boy), and better off with emulating the PC version. The MacOS game API is not set in stone like WinXP32/DirectX9c, the emulation options for PC games will keep getting better while your old Mac UT2004 discs gather dust.
SteamPlay is a great system to buy both sides at once. I have Borderlands 2 installed twice for Native and Parallels because the two platforms can't play together (lol version skew). Clasically, porting a game to Mac meant that the porter had to pay for rights up front, rights per copy, and Mac always got stuck with something expensive and late. Aspyr seems to have gotten a deal where they get a piece of the unified sales of BL2, Mac customers get a better deal/experience, and everybody wins. Feral needs to pay attention here, a $30 premium on a year old game doesn't cut it anymore. Perhaps selling either/or platform on Steam, and upping it to both with a $15 DLC would be acceptable. The straight SteamPlay sale is best for the customer, I don't care how you split revenue and will bend my purchases to whomever figures this out first. Aspyr has a head start (hint: I would buy BI right now if they promised SteamPlay).
