I am making this thread a collection of TF2 online resources, so you don't have to search around the forums. [size=20][b]Post any edits that you think that I should make or things I should add/remove[/b][/size], and I'll put them in the original post.

I'm not familiar with leagues outside of NA, so anybody that knows more, post here with a brief overview please.

[b]General Resources[/b]

http://tf2movies.org/ - nTraum - A collaborative online catalogue for competitive TF2 movies.
http://jump.tf/ - Rokio and Powerjohn - A website full of jumping content and tutorials.
http://comp.tf/ - Kaneco - A wiki dedicated to competitive strategy.
http://tf2maps.net/ - A website where almost every map is available for download, and for sharing and receiving feedback on custom maps.
http://fakkelbrigade.eu/maps/ - A huge collection of standard maps.

[b]Personal Customization Resources[/b]

http://tf2huds.net/ - omni - A collection of HUD releases that are currently updated/commonly used.
http://tf2cfg.info/ - Fresh'z - An online config generator that lets you customize everything that you want for your config. Contains 5 Preset Configs, an Easymode Config if you just want to change basic things, and an Advanced Config option that lets you change every single thing.
http://clugu.com/tf2mate/ - compton - A website for customizing class, xhair, autoexec, and fps cfgs.
http://tf2dingalings.com/ - A website where you can download a wide variety of custom hitsounds.
http://doodlesstuff.com/?tutorial=tf2hud - A concise guide to editing and personalizing a TF2 HUD.

[b]Server Customization Resources[/b]

http://logs.tf/ or http://sizzlingstats.com/ - Zoob/Technosex and SizzlingCalamri - Stats plugins for servers that allow you to view many statistics of any match played in tournament mode.
http://whitelist.tf/ - Wiethoofd - A website that allows you you create personalized whitelists, or to download whitelists from different leagues to implement.
http://serveme.tf/ - [url=http://serveme.tf/credits]http://serveme.tf/credits[/url] - A website that allows you to reserve a server for anything vaguely related to competitive TF2, completely free of charge, although donations are appreciated. 1v1 practice, private clan practice, pcw's, scrims, mixes, lobbies or officials. Basically anything that's not a pub or MvM match.
https://www.tragicservers.com/ - A website that you can rent cheap effective servers from. 12 slot servers for about $9.00/month.

[b]Practice[/b]

These are some good sources of practice in a competitive format without having to be a part of a team.

[size=20]Before playing, make sure you download Mumble, the preferred method of in-game communication of the TF2 Community at http://mumble.sourceforge.net/[/size].

[i]Tip: meds are usually the hold-up. If you're pretty comfortable with the medicine man, queue up to get some games started. As people get to know you for being decent/nice, you'll get picked for other classes, too.[/i]

http://tf2pickup.net/ - A website for playing pickup games in a competitive format.
http://rc.tf2center.com/ - A website for beginner lobbies in a competitive format.

[i]You can also connect to a variety of other sources with an irc program, or use ATF2 in your browser. They're generally higher skill than the lobby sites and require mumble (which they provide) during games.[/i]

#tf2mix - Friendly to newer players (but most people have played 6v6 a fair amount still) because it alternates Captain/automatic picking; if you aren't picked the first game, you're automatically put into the second.
#tf2.pug.na - High skill pugs, all games are Captain picks so it may take some time to get in, often starts late at night.
#tf2.mix.nahl/#tf2.pug.nahl - Highlander versions of the aforementioned.
#tf2.mix.eu - EU equivalent of tf2mix.
#tf2.mix.euhl - EU equivalent of tf2.mix.nahl.

[b]Recruitment[/b]

Finally, after doing all those lobbies, mixes, configs, and after having a comfortable grasp on game basics like movement and dm, you want to play on a team but aren't sure how to find one? Make a post with your experience, the classes you play, and when you're available to scrim/play games.

http://www.ugcleague.com/forum/forumdisplay.php?5-TF2-Recruiting - for UGC Highlander, 6v6, and 4v4 across all divisions. If you're just starting out, try here first!
http://etf2l.org/recruitment/ - Pretty much the same as UGC, but for EU.
http://teamfortress.tv/forum/4/recruitment - A recruitment forum for any league, but is more popular for high level 6v6 leagues such as ESEA and CEVO. If you've played a few seasons and are looking for a team, you'll have more luck here.

[b]Leagues[/b]

http://www.ugcleague.com/ - Free to play competitive league for Highlander, 6v6, and 4v4; many skill divisions so a good place to start your comp career!
http://etf2l.org/ - Free to play competitive league for Highlander, 6v6,there are many skill divisions, EU alternative to UGC.
http://comp.tf/ - Hosts a wide variety TF2 events and tournaments.
http://cevo.com/game/tf2/ - Paid 6v6 league with two skill divisions, becoming more developed and popular.
http://play.esea.net/index.php?s=league&d=standings - paid 6v6 league, high skill ceiling and LAN finals at highest level.
http://asiafortress.com/ - pending
http://ozfortress.com/ - pending

[i]Tip: CEVO and ESEA have demos available for download on match pages. these can be good to review and see how players at higher leagues play.[/i]

[i]Tip: use #tf2scrim to find scrims for your team, usually ESEA/CEVO teams looking for that week's map. State your skill level, what map you're looking for (or that you'll play any), and y/our server (if you have one, need to play on theirs, or don't care either way).[/i]

[b]Trading[/b]

For all you traders out there, these are some valuable resources to check going prices of items and to buy and sell items.

http://backpack.tf/ - A community-voted pricing site (so it's more fair) for all TF2 items; prices listed here are generally considered the "standard" value.
http://www.tf2outpost.com/ - Post to buy/sell your items. Check backpack.tf first, so you know what a fair price is! Also, be very careful with anyone adding you if you post a sale— there are a fair amount of scammers with phishing links.
http://scrap.tf/ - Add to the que to have automated bots buy your keys/metal/hats/weapons in return for metal/keys. It is more for low-ticket items like regular ol' hats, or converting any 2 weapons into scrap when they're not for the same class.
http://www.tf2wh.com/ - A site where you can buy and sell TF2 virtual items efficiently and safely through the use of a fleet of automated bots. Sells items for credits, which are only useable on the website to buy items.
http://bazaar.tf/ - An alternative to TF2 Outpost. Search among different trading sites if the item you want is not available on other sites.
http://www.trade.tf/ - A similar website to backpack.tf with an option to keep track of every new item added to the game on any update.
http://stats.tf/ - A website used to track items statistics of almost every kind, such as number of items, levels, paints, previous owners, and so forth.

[b]Useful Threads on This Site[/b]

http://teamfortress.tv/thread/183/collection-of-custom-huds - HUDs
http://teamfortress.tv/thread/7639/fps-improvement-compilation - FPS Configs
http://teamfortress.tv/thread/1261/fogs-custom-hud-crosshairs-version-2-0-released - Custom Crosshairs

[b]Bonus Demo Collection[/b]

http://teamfortress.tv/thread/1988/?page=1 - ESEA Season 12 demos
http://www.mediafire.com/download/vdqwu36sphixzw6/ESEA+S12+LAN+Package.rar - ESEA Season 12 LAN demos
http://teamfortress.tv/thread/7416/?page=1 - ESEA Season 13 LAN demos
http://etf2l.org/forum/lan/topic-27030/ - i49 LAN demos
http://teamfortress.tv/thread/15546/?page=1 - ESEA Season 15 demos