I've been using web-based email (squirrel mail derivative) as my primary email for several years because I love having my day-to-day office accessible online.
Now I've migrated to Gmail in an effort to consolidate and simplify.
The best resource I've found so far for Gmail hacks is Lifehacker's Hack Attack: Become a Gmail master. It seems from my initial reading that true Gmastery springs as much from the philosophy of email management as from the technical tools.
I'm interested in anything anybody has found to add to what Lifehacker suggests, via comment or Gmail.