qpdfview
qpdfview – free and open source software for viewing PDF documents with tab support
Okular
Okular is a free and open source, multi-platform document viewer that supports a variety of formats
Evince
Evince is a free and open source program for viewing PDF documents, comic books, and others.
Сhoqok
Choqok is a free and open source microblogging client by the KDE community
Cawbird
Cawbird is a free and open source, modern and lightweight Twitter client
Makagiga
Makagiga is a free and open source multi-platform simple software for creating to-do lists, notes, and reading RSS feeds
Purple Task
Purple Task is a free and open source simple software for managing tasks
OpenTodoList
OpenTodoList is a free, simple, cross-platform, open source software for creating notes and task lists
Planner
Planner – free and open source software for tracking your tasks, projects, and goals for Linux