I generally use Microsoft Digital Image Suite or Photoshop to tweak and edit photos. I recently downloaded and used Google's Picasa 3 and I love it. It's very easy to use and I especially love the Collage feature.
If you're new to digital imaging, try Picasa 3. It's fun, easy, and FREE to download.
Here's a fun example of the Collage feature using garden photos from Chile's garden.