This small addon adds search capability to the combat log.

How to Use

A search button is added to the top-right corner of your combat log.
Click this button to open a search box.
Type in text you want to find and hit [Enter].
The combat log will be used to display search results and how far back the log goes.
Dismissing the search box will restore the combat log to its previous state.

Notes

This addon does not monitor or store your combat log at all. It simply uses the history the default UI has already collected. Once the button is placed on the combat log there are no events registered and it's effectively inert until clicked.
The default log does not hold a huge amount of history. Don't expect to use this to compare what a boss did several pulls ago in a 25-man raid. It's more for spur-of-the-moment searches, like finding out who's casting swapblaster or aspect of the pack, or did that spell really hit me for a million damage.
In casual testing, I've had history go back over 60k log entries in one 25-man LFR boss fight and 40k entries in another, both around 5 minutes, though the data persisted for over an hour after leaving the raid. In a 5-man group the "duration" of data was about the same but only about 7k entries. Again, it persisted after leaving the group. I won't pretend to understand how this works. Just know that the history is finite.
Searches will ignore active filters when looking through your combat log history. If your log is set to 'My Actions' it will dig through everyone's actions too.
Search will only report the first 100 matches it finds. After that it will stop looking and just say it found 100+ matches.
The combat log's "Use Verbose Mode" filter setting changes the phrasing of the log ("faded" becomes "fades", "applied" becomes "gains", "You" becomes your character's name, etc). This addon will format/search the log using the same verbose setting as your currently active filter.

Combat Log Search