FileViewer.com

I'd really like the ability to have filters, where I can say only show lines with, or without certain strings, preferably definable with regular expressions, though that would slow it down I'm sure. And have multiple filters. And then be able to save the output to another file.

I'd really like the ability to have filters, where I can say only show lines with, or without certain strings

This is on the todo list - but probably won't be available until V16.

Note that you can save all lines containing a matching expression with the current version as follows:

Search for the string/regular expression while viewing the file and press the Bookmark button (instead of Search). If you are using the search bar, you can right click on the down arrow button and select Bookmark All.

This will bookmark every line that contains the string/regular expression. You can then copy them to a file by selecting Bookmarks->Save Bookmarked Lines from the Edit menu.