SurfFind is a free portable tool which searches text files for the keywords you specify.
The interface looks a little cluttered, with plenty of boxes and buttons, but a Help tab explains the basics. At a minimum you must choose the starting folders, the file types to search, and enter a keyword. You can also run case or whole-word searches, use regular expressions, specify multiple starting folders (C:\Here|D:\There), define the encoding to use when reading files, and use an Exclude filter to exclude particular wildcard patterns from the search.
Whatever you're doing, any matches are quickly listed in a table, with the file name, the line and column number for each keyword instance, and the sentence where it appears.
If you need more, clicking an entry in the table displays the file for you, with the keyword highlighted, or you can double-click the entry to open it in a viewer (Notepad by default, although you can set this to whatever you like).
You're able to run more searches and display the results in separate tabs, making it easy to compare the results later.
Your last few searches are also preserved in a "Recent" list, allowing you to re-run them in a couple of clicks.
SurfFind can't filter files by size or date, but has plenty of other advantages - tabbed interface, encoding control, multiple starting folders, exclude filter, keyword highlighting, more - and overall it's a likeable program.