Exchange files between 2 Macs over wifi

AirDrop

Open Finder on both Macs, on the left you'll see the AirDrop icon with the parachute and box. Click that on both macs and you should see each other. Then just drag files to the other computer's picture.

You don't even have to be on a wifi network at all, you just have to have wifi enabled on both Macs.

Edit: As mentioned below, not all Macs have AirDrop. The next best solution is to choose "Create Network" under the wifi menu on the Menu Bar (or join the same wifi network on both machines).

Then enable file sharing.

Now open finder and choose "network" on the left of a finder window, or in the top most folder (ctrl-click on the title at the top of the finder window, mine's called "Lance's MacbookPro") you should see "network" there.

If your Macs are on the same network and the one you want to connect to has File Sharing enabled you should just be able to connect to one from the other in the sidebar under the shared menu. After you connected and select which volumn to mount it will mount it as a volume on the Mac you are using and then you can drag and drop to or from that volume.

Airdrop is not available on all Lion installs, it depends on the age of the Mac, see OS X Lion: Can I use AirDrop with my computer?

Dropbox

Try Dropbox. It has LAN support (which Airdrop operates on) and a filesystem, so if you register an account, download the client for Mac onto both machines, setup the Dropbox folder to be somewhere handy (e.g. Desktop, where you can just drag-drop) it would be at least as speedy as Airdrop, and far more versatile, as it supports online uploading and more operating systems. Only limit is the 2GB of free folder space, which you can either expand via referrals (to 8GB) or by buying more space.