Julieanne Kost reminded me of options that surprisingly few people know:

Use either of these shortcuts to cycle through open, tabbed documents in Photoshop:

• Command + ~ (tilde) (Mac) | Control + ~ (tilda) (Win)
• Command + Tab (this is the same shortcut for both platforms).

Adding the Shift key to either shortcut will reverse direction.

When we changed the meaning of Cmd-~ and Cmd-1 in CS4 to be more platform- and app-consistent, longtime Photoshop users flipped out, and I didn’t blame them. As I wrote then,

By and large, keyboard shortcut changes suck.  Mature tools are like musical instruments, and you don’t go moving the piano keys or cello strings without a great need to do so.  It’s painful.  We know.

I still think those were the right changes, and if you prefer the older way, you can just choose Edit->Keyboard Shortcuts, then enable “Use Legacy Channel Shortcuts.”