Top 10, >=4 characters:
I weep for humanity :)
Russ Cox wrote the old Google Code Search, codesearch is an implementation in Go based on the same ideas.
So they can efficiently answer questions like "Give me all documents containing the word 'Linus'", but not necessarily "All documents containing the string '#def'".
That said, I am indexing -- I have my own custom backend that stores an in-memory index that lets me do arbitrary substring search (and more complicated queries, such as most character classes) much faster than a full search.
That said, the backend will fall back on a full regex search if necessary.