You can hold ⌥ (Option) to make windows resize symmetrically, from the center. It even works when resizing from any corner in Lion. Update: You can also hold ⇧ (Shift) to resize a window proportionally.

As far as I can tell this is brand new in Lion, the first version of OS X that allowed resizing windows from any side.

To enable hidden files/folders in finder windows:

Open Finder
Open the Utilities folder
Open a terminal window
Copy and paste the following line in:
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defaults write com.apple.Finder AppleShowAllFiles YES
Press return
Now hold ‘alt’ on the keyboard and right click on the Finder icon
Click on Relaunch
You should find you will now be able to see any hidden files or folders. One you are done, perform the steps above however, replace the terminal command in step 4 with:
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defaults write com.apple.Finder AppleShowAllFiles NO

By forcing us to adopt the two-column approach imposed by the skeuomorphic design, Apple is effectively deprecating groups as a feature. They are still there, but using them has just become much more painful. You cannot see groups or select one at all while you are viewing the contact details of a specific card. You first have to click on the red bookmark button at the top:

Before Lion it was possible to run an external display off a laptop and have the internal display disabled, even if you opened the lid. This can be useful for a myriad of reason including energy saving and better wifi reception. With Lion the internal display will always turn on when the lid is opened, even if there is already an external display connected.

A solution was posted on the Apple support forums by user chenga.8.

To go back to pre-Lion behaviour enter the following command in Terminal:

sudo nvram boot-args="iog=0x0"

To undo this change type type the following command or zap the PRAM (press Cmd+Opt+p+r at power up):

sudo nvram -d boot-args

You open up the PSD in Preview, make a minor edit to it such as adding a little text to it or draw a couple of red lines. The intention is to select all, copy, and go to your email app to paste it into a message. You'll just undo the changes or close it without saving later on.

In the background, when you left Preview it autosaved the file and destroyed all the layers. You even quit Preview without thinking, so you didn't get a chance to decline saving it.

Now, you could then reopen the image in Preview and use the "Revert Document…" menu or you could use Time Machine to get an older version back, but wow that's annoying.

Q: I want to show the Calendars sidebar all the time as I could in previous versions of iCal. Can I?
A: Nope. Cupertino thinks you don’t need to see this. Sorry.

The recently released Mac OS X 10.7 “Lion” (read my review of it at The Guardian) includes a new version of Mail, Apple’s desktop email client. There are several new features in Mail that I’m enjoying, and one of the least promoted is the new Favorites Bar. I think it’s very useful, particularly if you like to use keyboard shortcuts as much as possible, and so I’ve written this brief article describing how it works.

On Lion only, to make recent backups available to Time Machine users while travelling or otherwise away from their backups, Time Machine may make "Local Snapshots" hourly.
These snapshots are made only on portable Macs, and are kept on the Mac's internal HD (and on any other disks/partitions being backed-up) for a few days, space permitting.