How I met your mother (HIMYM) is a sitcom. The main character is Ted Mosby (Josh Radnor), who in the year 2030 recounts to his son and daughter the events that led to his meeting their mother. Ted's friends are Marshall Eriksen (Jason Segal), Lily Aldrin (Alyson Hannigan), Barney Stinson (Neil Patrik Harris, aka NPH), and Robin Scherbatsky (Cobie Smulders).
The show premiered on September 19th, 2005. Airs on CBS on Mondays, 8:00 PM. Official website.
Please note: I have listed each community under only one category. Many of them have several other purposes as well, so check their info pages. Also, I've only included the comms that have been active within the last year and have at least one post.
Overview at crack_van: here
himym_style (The styles/clothes of HIMYM)
himym_awards (HIMYM Awards for the best fanfic)
himym_slapbet (everything but Barney/Robin)
himym_animoods (animated mood themes)
daily_nph (daily NPH pictures)
daily_himym (daily HIMYM pictures)
dailyhowimet (daily HIMYM pictures)
Pairing specific comms
she_smulders (Cobie Smulders)
robinsmulders (Robin Scherbatsky & Cobie Smulders)
haveyoumetjosh (Josh Radnor)
alysonhannigan (Alysson Hannigan)
slutup (Barney Stinson & NPH)
Barney/Robin - you give love a bad name by damalur
Barney/Lily - the former Mr. and Mrs. Stinson by otempora42
Recs at crack_van
Fic recs @ TV Tropes
Have you met Ted? (blog)
HIMYM @ fanfiction.net
If I've missed any, please comment :)