Depending on your skill level.. Lightroom is a great Adobe app to organize and tweak your photos for a DSLR. CS4 has a much steeper learning curve but is probably the best app for professional quality photos. A cheaper option would be Paint Shop Pro, it has almost as many features as CS4 for editing. It's only about $50 online for PSP X2 version vs $400+ for CS4.
There are other free simple apps like Picasa 3 for sorting and organizing photos. Not that great for detailed editing, though. irfanview is a great free app for editing.