How can I do a full text search of plaintext files in my DropBox folder?
I often do such search, but I do not use fancy GUIs for that. If you are interested, then read on.

Unix has a great tool called grep which will search for patterns within files. It even supports regular expressions if you use egrep.

Example searching the word "hello" with all case mixture in the Dropbox folder:

cd /Users/name/Dropbox
grep -Riw hello *


That is it. Oh, I forgot to mention you have to use the Terminal application before typing these commands.

If you look for two words "hello" or "bye", you can do:

egrep -Riw "hello|bye" *


If you are looking for every words that contains "ell" (such as hello), you can do:

grep -Ri ell *


And finally (but read the manual page if you want more), matching every file that have the word "iOS" with this particular case:

grep -Rw iOS *