Gmail - tagging

Gmail is pretty good as it is. But, how about they let us tag email threads with some keywords. It's not that hard - you could always forward one of the messages to yourself and put the tag words, but they could make it easier. Say you are looking for some information. You do a search, but don't find it. Then, you remember something and do a different search and then you find it. It would be nice to now be able to tag this thread you just found with a couple of words that would make it show up with the first search. Wouldn't it?