How do I change the plymouth bootscreen?

With regards to Plymouth --

Are there alternative boot screens available?

What's the easiest way to change the boot screen?

You can get a list of themes available via the repositories through Synaptic Package Manager or through the command line via aptitude:

$ aptitude search plymouth-theme

p   lubuntu-plymouth-theme        - plymouth theme for Lubuntu
p   plymouth-theme-fade-in        - graphical boot animation and logger - fade-in theme
p   plymouth-theme-glow           - graphical boot animation and logger - glow theme
i   plymouth-theme-kubuntu-logo   - graphical boot animation and logger - kubuntu-logo theme
p   plymouth-theme-sabily         - plymouth theme for Sabily
p   plymouth-theme-script         - graphical boot animation and logger - script theme
p   plymouth-theme-solar          - graphical boot animation and logger - solar theme
p   plymouth-theme-spinfinity     - graphical boot animation and logger - spinfinity theme
p   plymouth-theme-text           - graphical boot animation and logger - text theme
c   plymouth-theme-ubuntu-logo    - graphical boot animation and logger - ubuntu-logo theme

i   plymouth-theme-ubuntu-text    - graphical boot animation and logger - ubuntu-logo theme
p   plymouth-theme-ubuntustudio   - Ubuntu Studio Plymouth theme
p   xubuntu-plymouth-theme        - Plymouth theme for Xubuntu

You can then install the resulting packages via Synaptic or aptitude as normal:

$ sudo aptitude install plymouth-theme-solar

If you're looking for something not in the repositories, UbuntuGeek has an excellent tutorial on how to install and create your own custom Plymouth themes.