So cunning you could brush your teeth with it!

I don't post recs very often, but this one deserves it: A Just Impediment by anon. It's a Doctor Who (Ten)/Blackadder (Two) crossover of magnificent proportions - the plot alone is worth reading, but it's hilarious, incorporates both canons, is totally in character, and features one wedding, two failed weddings, all the wonders of the universe and the threat of a sausage casserole, not to mention the best ending I've read in a long, long time.

Go! Read!