Utrecht Institute of Linguistics has produced a well designed lexicon of linguistics that is searchable online. Since the entries have been submitted by various users of the lexicon, there is a natural variation in the quality of the articles.
The Summer Institute of Linguistics' glossary of linguistics terms is somewhat more limited in scope than the Utrecht Institute Lexicon, but is still very useful. It contains only linguistics terms that could serve as glosses in a text, and excludes broader terms that designate, for example, theories or other entities that could not serve as glosses for words or phrases in a text.
Online Lexica / Dictionaries for Linguistics Terms