Uncomplex has released Mailplane 3.0.1, a maintenance release to the Gmail-specific mail client that revises keyboard shortcuts for opening mail due to a change that enables shortcuts to work in reply mode. The previous Mail > Open option has been revised to Mail > Open Conversation (Command-]), while Mail > Open In New Window has been added (Command-O). The update also lets you hide the tab bar, fixes an AppleScript issue with paragraphs in the mail body, squashes a bug with downloaded attachments containing special characters in their file names, and ensures that opening a file dialog no longer crashes the app in Mac OS X 10.7 Lion. ($24.95 new, free update, 20-percent-off upgrade discount or free upgrades for purchases after 1 October 2012, 3.6 MB, release notes)
Go Back and Forth Fast in Preview
If you're reading a PDF in Apple's Preview software, and you follow a bookmark or an internal link to move around within the PDF, you can quickly return to where you were by pressing the keyboard shortcut Command-[ (that's Command-Left Bracket). Or, you can choose Go > Back.
The command works iteratively, so you can go back to just the previously viewed page or if you issue the command again, to the page before that, and so on. There's also an equivalent Go > Forward (Command-]).