Welcome to the site!
First things first, I would spend some hours reading everything I could about the Fascinate and Android operating system. Just read it all!! There's a lot of very, very good information here.
Second, please do not install either a task killer or an anti virus program. Both types will end up causing more problems than they could ever solve.
Apps are a very personal thing. It all depends on what you need to do with your phone. Here's my list:
K-9 email app. It is much, much better than the stock email app.
Solitaire or Angry Birds - you've got to have a time killer for when you have to wait on someone/something.
MixZing - it is a much better music app than the native one.
Bedside - it is a clock program for when you don't want to be bothered by the phone/emails/texts/etc. It will turn your phone into a clock at night. It will still receive everything, but it will turn off notifications and you can let the phone receive calls or not receive calls.
5X Pro Zoom - a better camera app.
Documents to Go (Paid) - let's me read, create, edit Microsoft Documents, Power Points and Excel spreadsheets and view PDF's.
Folder Organizer - let's me change the Icons for apps that I put on the various home screens. Also let's me change the caption for those same icons.
Play around with apps and you'll find ones that work for you. Remember you can for paid apps you can buy them and you have 24 hours to uninstall them with no charge.