Well, you haven't mentioned any preferred fandoms, categories or genres yet, so it's difficult to make suggestions you'd want to read. But since you seem to be happy with a HP/SG1 crossover, I'll start with a few stories from either (or both) of those fandoms that I believe are worth a read.
Harry Potter/Firefly =
Browncoat, Green Eyes by
nonjon (
Finished, almost 300k words. I've said it often enough whenever this story has been mentioned, but it's one of the very few crossovers between Harry Potter and Firefly worth reading and certainly the only novel-length one. In this story, Harry Potter binds himself to the Weasley family as a guardian to be summoned in times of need. You can imagine that Harry won't be happy when the one doing the summoning calls himself Weatherby, right? This fanfiction also features a nice back-story about the Earth-that-was and how Reavers came to be.)
Harry Potter/Stargate SG-1 =
Harry Potter: An Ancient's Journey by
keiranhalcyon2010 (
Incomplete, ~400k words. This story was one of the first crossovers between Harry Potter and Stargate SG-1 that have been worth reading. In the past five plus years, it has grown to a respectable story the length of several novels.)
Harry Potter/Stargate SG-1 =
Fighting the Gods by
phoenix catcher (
Incomplete, ~300k words. The premise is very close to the previous story with very similar circumstances. It too hasn't been updated in more than a year, but what has been written, is readable. The writer has published a number of other crossovers with the Harry Potter character, but most of those are slash stories and that's not everyone's cup of tea.)
Harry Potter/Stargate SG-1 =
The Astria Porta Project by
Solunvar (
Finished, ~40k words. After Harry Potter has done away with Voldemort, he needs a new hobby, of course. This story is a surprising sequence of events, that begins with a treasure hunting Harry and continues with him opening a museum in the muggle world. How could that come to be a Stargate crossover? Go figure!)
Harry Potter/Star Trek/Star Wars =
Harry Potter and the Four Founders by
Darth Marrs (
Finished, ~130k words. The first story deals solely with the Harry Potter fandom, but there are two sequels,
The Forever Mage with Star Trek and the
The Lord of Light with Star Wars. In the first part of this trilogy, Harry is up against his arch-nemesis Voldemort and his wives - yes it's something of a harem story - are the reason why there are two more sequels. I wouldn't say that those are the best stories or crossovers ever, but they can make for a fun casual read. If you're interested in exploring the Star Trek backstory of the 21st century, you should give "The Forever Mage" a read.)
Harry Potter =
The Brave New World by
bellerophon30 (
Finished, almost 900k words, so it can be a bit of a chore to read. It's one of those stories that writers in the Harry Potter fandom have tried often and only seldom gotten right, or finished for that matter. In the Brave New World, Harry has enough of Britain, Hogwarts and Dumbledore and wants to try attending a new school in the good old US of A. Of course, trouble won't stay behind. I like this story, because the writer spends a lot of time on portraying the American wizarding world.)
Harry Potter =
The Lie I've Lived by
jbern (
Finished, ~200k words, but sequel yet to be written. In my opinion, jbern is one of the best writers of the Harry Potter fandom and this story is only one of several that could be mentioned. You should definitely check out his other stories like "Bungle in the Jungle", its sequel and "To Fight The Coming Darkness". In this story, Harry comes to an earth-shaking realization that will trouble him throughout his fourth year at Hogwarts. jbern describes a Harry that can be more than be led around by the nose and he's certainly a more realistic and life-like character than in canon.)
Harry Potter =
Poison Pen by
GenkaiFan (
Finished, ~75k words. It's Harry's 5th year at Hogwarts and he's fed up with the status quo. He gains a more slytherin outlook on his life and decides to affect some changes, both in the school and the greater wizarding world. Not that anyone else knows, of course

)
Harry Potter = Partners by
muggledad (
Finished, ~170k words. Partners is one of those stories that I appreciate both for its realism and for its escapism. A literally beaten down Harry escapes his relatives for the last time and receives help from the most unlikeliest places imaginable. The Harry Potter/Daphne Greengrass pairing has become a bit of a cliché, but this story is absolutely worth reading if you like Harry a bit more independent than in the books.)
Smallville =
The Portal by
a-delacroix (
Finished, ~160k words. This story will become ten years old this coming November and I like it so much for three reasons. First, it offers a nice flashback into the early seasons, when the Smallville fandom was still alright and the show was fun to watch. Second, the character of Chloe Sullivan has always been much undervalued in the show and this story fixes it neatly. Third, we not only see a true friendship between Lex and Clark but also a few glimpses into Ancient Rome. There's an even longer sequel of ~350k words called
Biological Families, where Clark once again travels into the past and meets not only Indiana Jones but also has to deal with plenty of Nazis.)
There's plenty more fanfiction that can be suggested, but if you're not looking for anything specific, then this forum won't be of much more help than any other. You see, Fanfiction.net already has communities for this sort of thing. If you go to any category on the website, you'll find a link called "Communities (123)" on the top of the page, where 123 denotes the number of user-created communities for this fandom or crossover pairing. A community is a list of stories that every reader can create. Much the same is true, if you open an author's profile page and check his or her favorite stories list. I'm sure that you'll always find something new to read for the next ten or twelve years between all those lists of recommended stories
