What is the maximum level that a character can reach in Borderlands for PC without any of the DLC installed, and with all of the DLC installed?
I have the Xbox version, but the maximum level without DLC should be 58. With "The Secret Armory of General Knoxx" the maximum is 69 (it goes to eleven, har har).
"The Zombie Island of Dr. Ned" doesn't affect the maximum level. "Mad Moxxi's Underdome Riot" doesn't either, but it does provide 2 extra skill points you only normally get by ranking up. One for beating the "lesser challenge" (only 5 rounds) in each of the 3 stadiums during playthrough #1, and the other for doing the same in playthrough #2.
The level cap for all characters was increased by 8 with the release of patch 1.4.1. This makes a total of 69 skill points to earn, though the four gained from DLC's 2 and 4 will still require those add-ons (as they are mission rewards). (source)
Gearbox has announced at PAX that they are releasing a free update to increase the max level cap by 8 levels and rebalance the vanilla game to accommodate for the increase. It is not necessary to own any of the previous DLC in order to take advantage of the level cap raise. So if you do not own the The Secret Armory of General Knoxx DLC, your max level cap will be 58, and if you do, the max level will be 69. 2 skill points will also be available in the upcoming Claptrap's New Robot Revolution DLC.