The gifting idea is probably one of the top reasons I'd go for the Orange Box rather than buy individual bits. I figure I'll gift the basic HL2 package to my brother since he loves Counter-Strike: Source.
Just for clarification, how exactly does the gifting process work? DO you choose which individual parts to gift or install?
STEAM keeps records of what you bought and all, once you activate the game's serial code. STEAM ties your games and their serial codes to your STEAM account -- even if you uninstall them or still have them installed on your PC, that serial code will always stick w/ your STEAM account.
On your "Games" list, you will see games you already own that you've even uninstalled are STILL listed under your account; they are listed under "Not Installed" portion at the bottom of your "Games" List -- y'know, so if you want to actually reinstall a STEAM game for some reason, it'll be absolutely no problem and no hassle to do so. It should be painless to reinstall them, since STEAM keeps your serial code tied w/ your STEAM account!
So, if you go and buy The Orange Box, say you ALREADY have activated to your STEAM account HL2: Original and HL2: Episode One -- whether it's still installed or not, that doesn't matter; STEAM ties ONE serial code to your STEAM account per game. Keep in mind when you buy Orange Box, you have ONE serial code for the entire game package; not for each game. So, here's what happens -- your EXTRA copies of HL2: Original and HL2: Episode One, they will be automatically listed and tagged w/ a "Gift" from STEAM. It'll list under your "Gifts" portion of all your games how many "gifts" you have to give out for each game.
"Gifts" last forever, until you decide to actually give it to someone.
About giving someone a "Gift", I ain't tried it -- but Pug already explained how to do it.