Lately I've just started replying to an email with a list of tags. That way my tag list ends up end up attached to the conversation and will show up in searches. It is a very fast and simple way to tag emails. The only shortcoming of this method is that you have to be really careful when you reply to the email that you replace the senders' email address with your own so you don't send your list of tags to them rather than yourself.. could be embarrassing (e.g. mytags: loser co-worker, annoying, haha...).

Also note that you can create a filter that recognizes your tagged emails and marks them as read so you don't get a new email when you tag your mail. The way to do this is to add to your email some unique text like "mytaglist:" before your tag list, then make a filter to mark any incoming emails with that tag as read.

Seems like it would be fairly easy to write a gmail plugin that would avoid the problem of accidentally sending the email to the last sender ... anyone want to take this on? :)