Here are my recommendations from February, organized by fandom.
Harry PotterA Bittersweet Potion and Procul His by Alchemia and Dent
Harry/Snape Slash
http://intertexius.com/series.shtmlSummary: Yes, this is the story where Harry turns into a cat and is adopted by Snape. At times funny, tender, and agonizingly heart-wrenching.
A Brighter Morning by GreenGecko (aka Salamander)
Gen
http://www.fanfiction.net/s/1846319/1/Summary: Harry Potter's son Arthur copes with a crushing burden.
A Year Like None Other by Aspen in the Sunlight
Gen
http://aalno.tauri.org/Summary: WiP warning. When tragedy strikes, Harry turns to Snape for comfort and protection. Already epic in length, with frequent updates.
"Contemporary Magical Innovations by H. Granger" by Kai
Snape/Harry slash
http://www.squidge.org/~kali/welcome.htmlSummary: A wonderfully sly tale of how Potter and Snape teamed up to win the war.
The Familiar by Resonant
Snape/Harry Slash
http://trickster.org/res/familiar.htmlSummary: "Quidditch" said the frog. Just go read it, okay?
On Wednesday by Jay Tryfanstone
Harry/Snape slash implied
http://individum.com/snarry/fiction/000788.phpSummary: Each day of Harry Potter's life is carefully planned...
SmallvilleInterstitial by Punk Maneuverability
Lex/Clark Slash
http://smallville.slashdom.com/archive/9/interstitial.htmlSummary: Clark isn't quite sure what's happening to him. Lex is the only one who understands.
Past Grief by Te
Lex/Clark Slash
http://teland.com/smallville.htmlSummary: After Clark's parents are killed, Lex is there to pick up the pieces. A dark vision of the future.
Pretty When You're Mine by Jenn
Lex/Clark Slash
http://seperis.illuminatedtext.com/smallville/pretty.html Summary: WiP warning. Brilliant AU starring hard-living party scene Lex and hooker Clark. Forget the cliches, this story will rip your heart out.
"SuperGlue" by Kat Reitz (Collaborative Evil) and Perryvic (Vicky)
Lex/Clark slash, other pairings, abuse
http://www.rpgplug.co.uk/Asylum/sv/glue.htmlSummary: After years of abuse Lex Luthor turned his back on his father and made a new life for himself as a CSI lab tech. But now Lex is transformed from lab tech into victim when a crime investigation brings up parts of his past that he's desperately tried to forget. See also rec for "Holding out for a hero" in the crossover section.
X-FilesMercury Falling by cslatton
Gen, violent content
http://fluky.gossamer.org (then search by author or title)
Summary: Stunning characterization and intricate details of FBI procedure drive this long angsty tale of Mulder in his Profiler days.
Oklahoma by Amperage and Livengoo
Gen, violent content
http://fluky.gossamer.org (then search by author or title)
Summary: A classic in the fandom. Profiler Mulder can't give up the hunt for a serial killer, even as the hunt threatens to cost him first his sanity and then his life.
Second Grace series by Logan
Mulder/Krycek Slash
http://www.hegalplace.com/logan/stories.htmlSummary: Krycek thought he'd moved on with his life, and then Mulder returns. A long series.
CrossoversSmallville/CSI"Holding out for a hero" by Kat Reitz (Collaborative Evil) and Tzigane
Lex/Clark slash, other pairings, abuse
http://www.rpgplug.co.uk/Asylum/sv/hofah.htmlSummary: After years of abuse Lex Luthor turned his back on his father and made a new life for himself as a CSI lab tech. But now Lex is transformed from tech to victim when a crime investigation brings up parts of his past that he's desperately tried to forget. This story and SuperGlue both share many of the same story elements, with slightly different twists.