Drawboard PDF, Virtual Tablet, Sling Player (if you use the Slingbox), SSH-RT if you need to terminal into 'NIX Servers or network gear, Gobble (Podcast Catcher), TWiT.tv, Plex Client if you use the Plex Service, Archsoft Sowbiz (Video Editor). Just to name a few... off the top of my head.
Plex - accessing my plex library on the SP2.
LCARS interface - pretty feature rich Star Trek interface emulator.
PIN Steam - allows me to create custom tiles for steam library games.
Everything else has been a game. Similar situation I have £12 to spend but not sure what on. Is your gift card specifically for Windows Store apps? Normally MS account credit can be used for Music/Video/Games/Xbox/Windows Phone as well as Windows Store if you still can't find much to spend it on.
Another vote for Plex.... IF you're running the Plex server. The Plex client for Windows, I'm running it on my Surface RT, is a very nice application and works wonderfully easily connecting to my Macbook Pro.