http://pixeljoint.com/forum/forum_posts.asp?TID=11299 - this one's good as a general all-round beginner's guide

http://www.kyrieru.com/p/this-first-part-will-cover-many-of.html - this one is really good if you're running into the problem of muddy sprites and pillow-like shading; if you're a beginner who knows there's something "off" about their work but not sure what, this could provide some useful pointers

https://derekyu.com/makegames/pixelart.html - walking you through the software/hardware/general know-how you need to put a character sprite together, helpful tools, and a guide of common mistakes to avoid making

https://forums.tigsource.com/index.php?topic=6745.0 - this page is about curbing ambition and being practical in your pixelart - how detailed you want it to be, how many colours you want to use, etc

http://gas13.ru/v3/tutorials/sywtbapa_almighty_grass_tile.php - a long and useful series, mostly on background graphics but also character sprites, discussing some little tricks of using small colour counts (taking pixels out of an outline or changing the outline colour to suggest details)

http://finalbossblues.com/thinking-in-color/ - this page talks about hues in your palette, useful to know

http://2dwillneverdie.com/tutorial/ - lots of incredibly useful information here, from turning drawings into sprites, to keeping your palette while painting your sprites, to subpixelling, lots of handy articles

There’s also an archive of some really good tutorials on anti-aliasing, fighter sprites, chibi fighter sprites, etc etc that were wiped off the internet years back, worth browsing through: https://mega.nz/#!n5oGhQCD!xMrDFFEwR0khHdNYjQvzXO9fu97G0STAevqeVszXT0c