It could be an email, a blog comment, a Facebook wall post, or (gasp) an actual, physical letter, with a stamp and everything. If you're feeling particularly bold and Rogueish, you could even make a phone call.
If there's someone out there you admire, someone whose work means something and speaks to you, let them know. Sometimes we're just so busy enjoying the articles, songs, books, etc, that we don't take the time to say so out loud.
And along with writing some fanmail, maybe you could find the time to tell a friend about whatever singer, artist, writer or other creative types you're enjoying these days.
Spread the word, share the love.