Software to create a video slideshow?

You can try imagination from the repository.

sudo apt-get install imagination

Imagination is a lightweight and user-friendly DVD slide show maker with a clean interface and few dependencies. It only requires the ffmpeg encoder to produce a movie to be burned with another application.

It currently features over 50 transition effects. Exporting the slideshow in FLV format is supported as well.

kdenlive works for me. You can install it from repo (sudo apt-get install kdenlive), but you can compile more codecs in with ffmpeg from source and kdenlive from source.

It looks a bit intimidating, but you don't need to know much to make a slideshow with it. First, put all the photos you want in a folder, then do Project → Add Slideshow Clip

and select that folder (there are some options for transitions and such there too).

Then, drag that clip down onto the timeline, e.g. to "Video 1", and click Render, pick a file name and format and Render to file.

If you also want music, do Project→Add Clip and pick a song and then drag it down to the timeline. Right click the audio clip at the end of the slideshow clip and Cut clip so you don't get audio after the slideshow is done. Then Render.

I for one really like Videoporama. The latest version comes with many effects, allows you to insert videos (or only parts of a video) too as well as audio (and edit audio).

Another similar tool is PhotoFilmStrip. It uses the "Ken Burns" effect for the pictures transitions and you can also add music and captions to your photos.