If you want a simple Plymouth manager to easily change, install and remove Plymouth themes, try the the new Zorin Splash Screen Manager.
Epidermis theme manager is an open source GTK application for managing, automatically downloading and installing themes of various types, in order to transform the look of your Ubuntu desktop, from the moment you turn it on until the moment you turn it off.
Gnome-Art Next Generation is the successor of Gnome-Art and lets you change your Gnome-themes (icon-, wallpaper-, splash-,...-themes) with one click. It is in an early stage and under development but it looks very promising already.
StartUp Manager, or SUM, is a gui tool for changing settings in Grub, Grub2 and Usplash.SUM should work with recent versions of Debian and Debian-based distributions such as Ubuntu.