Wacom Components for the touchscreen would be nice, but they don't make any sensor boards in the right size.
Lipi Toolkit "(LipiTk) is a generic toolkit for online Handwriting Recognition (HWR), and packages HWR tools and algorithms with standard ink representations and HWR application interfaces. The toolkit is designed to support different sets of users, with different requirements. Released under a dual MIT/GPL license."
HRE API A Portable Handwriting Recognition Engine Interface from Sun Microsystems, under GPL 1.0. Ported to Linux in 2000. No work since then it looks like, but could still be useful.
Xournal is an application for notetaking, sketching, keeping a journal using a stylus. It is free software (GNU GPL) and runs on Linux (recent distributions) and other GTK+/Gnome platforms. It is similar to Microsoft Windows Journal or to other alternatives such as Jarnal, Gournal, and NoteLab.
Rosetta - Multistroke / full word handwriting recognition for X
Calligrapher Commercial, for PocketPC, but supposedly quite good