I just did this with my 2 laptops. Easy way to transfer your purchases without re-downloading.
Navigate to "~user/Music/itunes/Mobile Applications" on the computer which has the applications already sync'd up. This is folder where your apps are stored when you sync.
Copy that folder onto a jump drive (or if you have the computer with you, just locate it in your shared section of finder), and drag and drop it onto the iTunes app of the computer you want to add the apps to. Viola! Transferred apps. Then it will ask you to authorize the computer (if you haven't done so already), and if you have multiple accounts of downloaded apps, it will prompt you one after the other...really convenient. I have 3 accounts. 1 old one from years ago, one when I bought my first Mac 4 years ago, and my .me account. Having bought things over the years in different accounts it makes that process of authorizing many users very simple instead of logging in and out to transfer purchases.