Tl;dr - I paid $10 for the pro version of TinySpell, although you may be happy with the gratis version. AsUtype has more options, but I only want the core, so couldn't justify paying four times as much (YkMMV).
No one else has answered yet, so here's what I have found so far.
My first thought was to code my own, and I still may.
Then I found asUtype, which looks good, and I may yet pay $39.99 to buy it.
For the time being, TinySpell looks great:
tinySpell (totally free - no adware/spyware/nagware/etc.)
Checks spelling in any Windows application
Checks spelling on-the-fly (alerts if the last typed word was misspelled)
Checks spelling of text that is copied to the clipboard
Allows you to specify applications for which tinySpell is disabled or enabled
Allows you to add words to the dictionary
Optionally beeps on error (beep sound can be easily set to any wav file)
Optionally displays a spelling tip (see example)
Opens replacements list with a simple mouse click or a hot-key
Optionally copies the selected replacement word to the clipboard
Optionally inserts the selected replacement word into the document
Provides easy access to on-line web services
(Search, Dictionary, Thesaurus, Encyclopedia) (see here)
Uses little system resources
Although I asked for gratis, I am going to spring $10 for the pro version, simply for the auto-correct feature.
tinySpell+ (in addition to the above features) Buy tinySpell+ Now
Supports Hunspell dictionaries in many languages (see Dictionaries below)
Remembers the last error (even if it is not the last typed word)
Optionally displays a spelling tip with instant correction (see example)
Includes a clipboard spell checker that shows all spelling errors in the
clipboard's text in a single glance, and allows you to correct them
Learns your spelling errors replacements and offers you better suggestions
Allows you to add auto-replacements to the dictionary
Allows you to add a list of words to the dictionary
A scheduled Import/Export of the words added to the dictionary allows easy
backup and synchronization of the dictionary with other users on a network
Automatically switches dictionary according to current language (see Dictionaries below)
Adds "Translation" to the on-line web services mentioned above
(Search, Dictionary, Thesaurus, Encyclopedia)
Allows you to easily choose the default on-line service in each category
from a list of services, or add new services to the category (see here)
Allows you to open a small bar on your desktop that lets you enter text
and search for on-line information (see here)
Provides the following additional options:
- Check Capitalization (proper nouns, etc.)
- Mixed CaSe is an error
- Ignore UPPERCASE words
- Ignore words with numbers
- Ignore Internet & email addresses and file names
- Ignore escape sequences (words that start with user specified escape characters)
I may still code my own, and make it pan-platform. Any suggestions for missing features?