I just have a minute, but here are a couple that spring to mind:
Wendy Richard's
At Wits End on the East End is a LnC/Doctor Who/EastEnders crossover
Marcus Roland has done quite a few LnC crossover fics, including some with Torchwood, Harry Potter, Buffy the Vampire Slayer, Highlander, Veronica Mars, etc. You can find his stories
here .
A few years ago, MrD8a issued a crossover challenge.
Here is the TOC.
Labrat already mentioned Timeless, which is fantastic.
Lynn wrote one last year called
The Case of the Flying Man , which is a Sherlock Holmes crossover.
Oh, there was a great Pushing Daisies crossover written a couple of years ago by IolantheAlias called
Pie Isn\'t All that Easy .
There are a couple of stories I remember that were started on the mbs, but were never finished - a House crossover and a Remington Steele crossover, but there are a bunch of stories posted to the archive. You can find a lot by searching by the keyword "crossover".
B