So I guess there’s a 4th way and equally easy.
Just curious what the official ‘1st’ and ‘2nd’ ways are? I mean, you’ve got:
1. Drag and drop from an open folder into PS
2. Ctrl-O
3. Mouse to File menu and select Open
4. Alt F O (using Windows keyboard shortcuts – no point in using this one though, as Ctrl-O is just as good and 1 key faster.)
5. Double-click the grey background of PS
(methods 2-5 require you to navigate to the file in the Open File dialog.)
6. If it’s one of the last 10 files you had open (or whatever you have the Recent File List number set to – 10 is default), you can just mouse to File/Open Recent and choose from the list.
7. If it’s one of the last 9 files, this is even hot-keyable: "Alt F R 1" will open the last file, "Alt F R 2" the file before that, and so on. Any more than 9 and there’s no hotkey for it, though you can use the arrows to select it then.
8. Use the File Browser (PS 7 and later)
9. If it’s a PSD file, or any other format you’ve got set to open PS by default, just double-clicking it will open it in PS. (Technically not from "within PS", but if PS is already open, it will just quickly open the file, rather than open a second copy of PS.)
10. Even if it’s not a format that’s set to open PS by default, you can usually right-click the file and select "Open with…" and choose PS.
I think that pretty much covers it…