A Vista-esque feature of OS X 10.5 is that it tags web downloads (not just those from Safari) as such, and then warns you about running downloaded apps. Archived (e.g. zipped) files inherit the tags from their tagged container. This is an annoyance to power users, but, being a power user, I can do something about it. ;)
Save the following script (original source) in ~/Library/Scripts/Folder Action Scripts. You'll probably need to create the Scripts directory and its subdirectory, e.g. with mkdir -p ~/Library/Scripts/Folder Action Scripts:
(*
"Unquarantine" by Henrik Nyh <http://henrik.nyh.se/2007/10/lift-the-leopard-download-quarantine>
This Folder Action handler is triggered whenever items are added to the attached folder.
It gets rid of Leopard's annoying "this application was downloaded from the Internet" warnings by stripping the "quarantine" property.
*)
on adding folder items to thisFolder after receiving addedItems
repeat with anItem in addedItems
set anItem's contents to (quoted form of POSIX path of (anItem as alias))
end repeat
set AppleScript's text item delimiters to " "
do shell script "xattr -d com.apple.quarantine " & (addedItems as text)
end adding folder items toMac OS X Hints
http://hints.macworld.com/article.php?story=20071029151619619