The last time I posted a 1TB external drive for less than $100, the deal vanished faster than Sarah Palin after the election (thank you, thank you, I'm here all week). Fortunately, another drive came along to take its place: Newegg has the Hammer Morespace 1TB USB 2.0 External Hard Drive on sale for $99.99 (after a $20 mail-in rebate). Shipping will run you about $11.
No, I didn't make up the name. And, no, I never heard of the Hammer brand before today. The reviews I've seen are mostly positive, but one concern is the drive enclosure's lack of a fan, which means you probably shouldn't run it 24-7. Also, it only has a one-year warranty, whereas some external drives are covered for three years.
The real deal-breaker for some will be the lack of an eSata interface. It's not like USB 2.0 is slow, but eSata is significantly faster. If your system has an external eSata port, you should definitely look for a drive that supports it.
Way back in the stone age, I sold build-to-order PCs. I distinctly remember the day that 250-megabyte drives dropped to $250, which seemed like a huge deal at the time ("A buck a megabyte!"). Now you can get a terabyte for $100. Quite extraordinary. Anyway, the rebate deal (PDF) expires November 30, so you've got time--assuming Newegg doesn't run out of drives.