Apologies if any of these have been recced before, I have a lousy memory!
Serenity, Nevada by Christina K.
http://www.fanfiction.net/s/4552093/1/Serenity_NevadaSummary: Oz's summer road trip takes him to a sleepy little town in Nevada- one that just woke up with a big new problem, and the local sheriff and citizens may have to face a reality they never expected before the closing credits. Crossover with Firefly.
It's a lovely AU romp in which Sheriff Malcolm Reynolds and his team have to deal with a sudden incursion of the supernatural. Fortunately they have a little help from a familiar Buffyverse character...
The Lassie Experiments by Nothorse
http://www.tthfanfic.org/Story-19201/Nothorse+The+Lassie+Experiments.htmSummary: Of course Rossum Industries experimented on animals.
Nice Dollhouse / Dr. Horrible crossover and origin story for Bad Horse.
Synth by AstridV, etothey
http://archiveofourown.org/works/8654Summary: Story by Yhlee, art by Astrid. An Angel/Fray crossover, set post-NFA, post-apocalypse... in which Angel and Gunn end up in Melaka Fray's Haddyn. (12 pages)
A very nicely drawn Angel / Fray crossover comic.
A Horrible String of Stories by ShyBob
http://www.tthfanfic.org/Series-1081Summary: Things that happened before, during, and after Dr. Horrible's Sing-along Blog.
This is actually a series of several Dr. Horrible stories; some comedy, some tragedy, and a South Park crossover. What's not to like?
Five Vampires Dr. Horrible Never Met by Snowpuppies
http://www.tthfanfic.org/Story-16912/Snowpuppies+Five+Vampires+Dr+Horrible+Never+Met.htmSummary: Dr. Horrible has a plan to become more evil. Unfortunately, he needs the help of a vampire. Contains character death (the funny kind) and implied character vamping.
Five Doctor Horrible / Buffyverse crossovers. Dr. H. decides that the only way to be more evil is to become a vampire. Predictably, it never ends well...
Hope you'll like them.