Adobe Photoshop CS4 The big dog but EXPENSIVE
Adobe Lightroom v2.0(update available for v2.3)
Adobe Photoshop Elements 7
Adobe FREE TRIALS
Gimp 2.6 (FREE)
Bibble 4.1 Pro ($159) & Bibble 4.1 Lite ($89)
Free Trials are availabe
Also, if you qualify for "Academic Status" there are very nice savings on some software...IE: Adobe Lightroom goes for around
$99 for the Academic Version(vs reg. $300).
EDIT: You might want to plan out how you download free trials...if you D/L them all at once you will end up with them all expiring at the same time...so if you want to stretch your "free" processing you might d/l CS4 first and when that expires d/l lightroom etc....also, with regards to adobe if they come out with a new version say Lightroom 3.0 and you did a trial with Lightroom 2.0 you will be able to do another 30 day free trial on the 3.0 version...now if they update the current version from say 2.3 to 2.4 and you have already used your 30 days you wont be able to do it again...just when the main version changes.