SpellMaster for the (6502-based) BBC Micro was seriously impressive given the space limitations.

It did both spell checking and correction (and had an anagram finder as a bonus), had integration with several different wordprocessors, check as you type functionality, AND its own integrated editor on top of that. The built in dictionary had a claimed 58k words (with a claimed checking speed of 10k words per minute). All of this was somehow squeezed into 128k (as a ROM on a carrier board with a hardware bank switching mechanism paging in 16K at once).