Just in time for the release of Borderlands 2, PlayStation Plus' "Instant Game Collection" will add the original Borderlands next month. PlayStation Plus owners will be able to download the game "for free" as part of their membership beginning September 4th. As usual, the game will remain accessible as long as you maintain your Plus subscription.
Plus subscriptions start at $17.99 for 3 months, $49.99 for a year.
In addition to the free game, PS+ members will also receive a 50% discount on all Borderlands DLC. The full list of discounts can be seen here. Catching up on the original game will also unlock goodies in the sequel, making it quite the bonus for anyone intending on playing the sequel.