I've been using Duotrope's Digest for just over a year now, and it has proved endlessly useful in the constant hunt for new fiction markets. However, it was only the other day that Alex Kane turned me on to their free submission tracker--only further proving that Duotrope is, truly, endlessly useful.
How I missed the tracker before I can't say, but it's... it's awesome. Just check it out for yourself. If nothing else, it is far better than the scraggly OpenOffice spreadsheet with which I used to track my submissions; that thing was so awful I finally abandoned it and tracked my submissions inside my head.
And yeah, that was a bad idea.
Anyway, the Duotrope tracker had me so excited I sent off an old manuscript, something that I wrote for Futurismic which was subsequently rejected there and elsewhere and then forgotten. I expect it will be a challenge to find a market for the story--a tongue-in-cheek, near-future blade-for-hire novelette written in mock British pidgin-slang--but it's a blast to send it out and track it--track it, that is, so dang effectively.