I think a truck with a looped rope and some grunt might be more effective at straightening them pillars than a tilt-shift - but I wouldn't want to be explaining to the cops; "no, no officer, I am pulling them vertical for my photo, not trying to steal them"
If you only intend to display on the web (and not print large in fine detail), a tilt-shift lens is probably not a worthwhile spend for a non-professional photographer, even one with architectural leanings (groan), because the softness introduced by perspective correction will be hidden in the down sizing and final sharpening.
The pillars here require a feature called "puppet-warp" in Photoshop CS5, not sure if GIMP, or any other free software has it.
I guess the manual way would be to;
copy them to three new (and separate) layers
rotate each of those layers in the necessary direction and angle to get each of 'em vertical
cut them out so only one pillar remains on each layer
on the main background image clone the leaning ones out entirely
now reveal each new pillar in the appropriate position
Good luck with that