Scite is a free open-source text editor based on the Scintilla source-code editing component. It is extremely powerful, though the documentation isn't as good as it could be. To help rectify this here are some tutorials and scripts to get you started.
Tutorials
Binding Commands A tutorial on how to bind Lua functions to the tools menu and keyboard shortcuts.
Word Count An introductory by example tutorial on using Lua, concentrating on getting information from the current text field. The completed script is available in the scripts section.
Selection Tutorial A tutorial on manipulating selections of highlighted text.
Scite Preferences An examination of some useful preferences and options.
Lua Scripts
Word Count A word count script that counts the number of characters, words and lines in a document.
Tag Complete An XML/XHTML script that autocompletes tags. If you type <tag>, </tag> will automatically be inserted. The script ignores <tag/>.
Reformat XML An XML/XHTML reformatter. It's a pretty crude script, useful when you get unformatted machine generated XML. Not as nice as hand-formatted code so be warned.
Scite Resources
Lua Reference Manual. If you're writing scripts you'll be making use of this reference a lot.
Scite Documentation. Scite is one of those programs where you get a lot more out of it if you read the documentation. Do so.
Example Scripts. A small number of scripts and an introductory tutorial.