For me, there's a lot of laziness involved, so it's usually Picasa for everything, normally spotting, some level or horizon adjustment, occasionally a shift to monochrome, and exporting back out to the web.
I'm rarely very happy adjusting levels in Picasa, and spotting is sometimes very nice and sometimes frustrating. I used the LightZone demo a while back and *loved* the way it handed adjusting levels and curves, especially for monochrome images, but I'm on a Mac and waiting for the open source version to come out of beta.
I have considered swapping over to LightZone and digiKam on either Mac or Linux, or just buying Aperture and calling it done. Also, if you haven't seen the blasphemy that is Windows 8 on Mac hardware, it runs extremely well on Macs that won't run OS X 10.8+, and unfortunately I've never been able to get the SilverFast demo to run well on OS X, and I keep hearing good things about it.