Currently I have the following as "App Tab"s in FireFox
Gmail for Email/Gchat, Mint.com for keeping an eye on my money & Hootsuite to keep up with Social Networks
Anything I'm working on is just a regular tab.
This keeps me focused on the tasks at hand, but reminds me to stay on top of email, money & social tasks.
I've gone back and forth between a few other pinned tabs, but have slimmed down to the 3 above to cut down on FireFox going insane on consuming memory + to keep me focused/
Curious what my fellow HNers have pinned.