In no particular order:
For what it's worth, I have unlimited data and also use wifi. It seems like I've been watching a lot of videos for entertainment.
Justin.TV - worth it to me because I can stream justin.TV with no commercials. I like Pawn Stars, Big Bang Theory, watching lifecasting etc.
The app doesn't freeze for me except occasionally, and I can easily restart it. To be fair, the app is legit, but some of the broadcasting on justin.tv is copyrighted material, so certain channels could be taken down overtime. I was using Streamie Pro app previously and the official justin.tv app works much better.
Titanium Backup Pro--for someone that likes to flash custom roms and load updates at my convenience, this app maks backing up and restoring apps so much easier
Swiftkey--I just prefer it as my default keyboard. I type traditionally, but if you like to trace, swype works well too.
Google Voice--use it as my default texting app and like how it can transcribe my voicemails.
Skype--If you use skype on your pc, you'll probably want it on android. I use it a lot for video calling
Apps I would also list, but you need a subscription, so I didn't include them in the top 5:
1. HBO to Go, Max to Go, Watch ESPN--if you have HBO, Cinemax, or ESPN as part of your TV or cable package, these apps make it really easy to watch on the go.