I don't have a Galaxy S or SII, but I have a Droid X. So I can suggest some apps

All the apps I suggest are free.
-Barcode Scanner (Scans bar codes and checks the prices of w/e you scanned online as well as showing you the website of the product. Haven't tried it with food, just headphones and what not. So not sure if you should get Redlaser too like pillar suggested)
-Battery Solo Widget (It's more or less just a gauge that tells you how much battery power you have with a number and a graphical representation. I find it easier than the little thing at the top)
-Battle.net Mobile Authenticator (I use it for WoW and SC2)
-Flashlight (Uses your LED light from your camera as a flashlight)
-Glow Hockey (Air hockey just everything glows, not the BEST game, but it's fun to play when waiting for your GF or w/e to get out of the bathroom)
-Lightning Bug (It just plays background noise, like water crashing against rocks at a beach, or rain and lightning. It's pretty neat. I use it when I sleep sometimes.)
-IMDb (REALLY useful)
-Stupid Zombies (Pretty fun game. Object is to kill all the zombies using only X amount of bullets. The bullets bounce off walls and what not.)
-Speed Test (App from
www.speedtest.net. I've used it to test what kind of speeds I get in different locations. I live in a town far away from a Verizon tower)